So, let's begin:
IS LBX2
<WeaponStats Health="15" slots="4" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="2" damage="1" heatdamage="0" impulse="0.04" heat="0.5" cooldown="0.72" ammoType="LB2-XACAmmo" ammoPerShot="1" tons="6" duration="0.0" lifetime="10.0" speed="1330" volleydelay="0" spread="0.35" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
Clan LBX2
<WeaponStats Health="7.5" slots="3" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="2" damage="1" heatdamage="0" impulse="0.04" heat="0.6" cooldown="0.72" ammoType="ClanLB2-XACAmmo" ammoPerShot="1" tons="5" duration="0.0" lifetime="10.0" speed="1330" volleydelay="0" spread="0.35" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
So, the core differences between the weapon (outside of base weapon crits+tonnage) is health (not really of any consequence), and the heat difference of .5 (IS) vs .6 (Clan).
Seriously?
There are two obvious problems with this weapon... projectile speed and spread.
Let's consider the AC2 (and indirectly UAC2):
IS AC2
<WeaponStats Health="10" slots="1" type="Ballistic" projectileclass="bullet" numFiring="1" damage="2" heatdamage="0" impulse="0.0375" heat="0.6" cooldown="0.72" ammoType="AC2Ammo" ammoPerShot="1" tons="6" duration="0.0" lifetime="10.0" speed="2000" volleydelay="0" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="1.0" critChanceIncrease="0.06,0.03,0.01"/>
The AC2's velocity is 2000. This is 50% more than the LBX2 velocity (or looking it from the other way, the LBX2 is only 2/3 or 66% of the AC2's velocity). What possible reason would you keep the LBX2 to be an inferior AC2 (or UAC2 for that matter)?
To put into context, the ERPPC has slightly less velocity than the LBX2/CLBX2.
As an aside, speaking of the ERPPC, there's very little reason to see use of the ERPPC over the CERPPC due to the damage... sure the heat on the IS ERPPC is better, but it's not enough to validate its use. Increasing velocity on the IS ERPPC would be an improvement (might as well make it more accurate if it doesn't deal more damage like the CERPPC).
I mention spread for a reason. One of the major changes between Clan SRMs and IS SRMs besides the obvious tonnage savings is spread. For Clans, this spread usually makes one generally need to commit to Artemis, particularly for the CSRM6, but can be questionable with the CSRM4. I don't understand why improved spread for the IS cannot be achieved.
As an aside, the SRM4+SRM2 are not very durable since the health changes. This is impacted further by using Artemis (remember, Artemis increases crit slot usage, thus making it easier to hit) and for whatever reason health was not scaled to compensate for that (for any SRM, Clan or IS for that matter).
Anyways, for the LBX2, spread probably makes little consequence, but it should still be tightened on the IS end, in additional to increasing velocity for both IS+Clan versions of the LBX2.
IS LBX5
<WeaponStats Health="15" slots="5" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="5" damage="1" heatdamage="0" impulse="0.04" heat="1.0" cooldown="1.66" ammoType="LB5-XACAmmo" ammoPerShot="1" tons="8" duration="0.0" lifetime="10.0" speed="1330" volleydelay="0" spread="0.65" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
Clan LBX5
<WeaponStats Health="7.5" slots="4" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="5" damage="1" heatdamage="0" impulse="0.04" heat="1.5" cooldown="1.66" ammoType="ClanLB5-XACAmmo" ammoPerShot="1" tons="7" duration="0.0" lifetime="10.0" speed="1330" volleydelay="0" spread="0.65" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
For whatever reason, it was determined that there's a heat difference of 1 (IS) vs 1.5 (Clan). That's it. Yet ironically it has the same velocity as the LBX2/CLBX2. Spread is the same, so I guess Clan tech is supposed to stay superior?
IS LBX10
<WeaponStats Health="15" slots="6" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="10" damage="1" heatdamage="0" impulse="0.04" heat="2.0" cooldown="2.5" ammoType="LB10-XACAmmo" ammoPerShot="1" tons="11" duration="0.0" lifetime="10.0" speed="1100" volleydelay="0" spread="0.9" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
Clan LBX10
<WeaponStats Health="10" slots="5" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="10" damage="1" heatdamage="0" impulse="0.04" heat="2.0" cooldown="2.5" ammoType="ClanLB10-XACAmmo" ammoPerShot="1" tons="10" duration="0.0" lifetime="10.0" speed="1100" volleydelay="0" spread="0.9" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
Here's my gripe. This is a pure copy and paste job that never got changed outside of the required changes (tonnage+crit). I'll leave it to other people to make guesses.
IS LBX20
<WeaponStats Health="15" slots="11" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="20" damage="1" heatdamage="0" impulse="0.04" heat="5.0" cooldown="4.0" ammoType="LB20-XACAmmo" ammoPerShot="1" tons="14" duration="0.0" lifetime="10.0" speed="1100" volleydelay="0" spread="1.0" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
Clan LBX20
<WeaponStats Health="16.5" slots="9" type="Ballistic" projectileclass="bullet" numFiring="1" numPerShot="20" damage="1" heatdamage="0" impulse="0.04" heat="6.0" cooldown="4" ammoType="ClanLB20-XACAmmo" ammoPerShot="1" tons="12" duration="0.0" lifetime="10.0" speed="1100" volleydelay="0" spread="1.0" gravity="0,0,-9.8" maxDepth="10.0" critDamMult="2.0" critChanceIncrease="0.14,0.08,0.03"/>
There's the obvious heat change 5 (IS) vs 6 (Clan), but nothing else. This deserves a much longer/bigger discussion.
The biggest problem with the LBX20 is kind of along the same issue that the Heavy Gauss is facing (but Heavy Gauss is a whole different beast).. it's competing with the AC20. This DOES NOT mean the AC20 needs nerfing - it's that the addition of the LFE makes the AC20 a much more attractive proposition at close range. In order to use the IS LBX20 (or Heavy Gauss), you'd have to commit to a slower/larger STD engine. Unlike the Heavy Gauss that has a handicap (in the form of "the CASE penalty" - you equip it for your own safety as a build penalty or risk it making your life miserable, unlike Clan mechs that have CASE sprinkled wherever), the LBX20 is very much not gaining additional benefits (better spread, damage, something) considering the build disadvantage. It's not to say Clan Omnimechs don't have disadvantages, but Clan Battlemechs avoid a lot of these build penalties to field the Clan LBX20.
Essentially as currently constituted, the IS LBX20 will be a pointless option until it is revamped/buffed.
Here's one other bit of Dakka that still needs to be looked at:
IS Gauss
<WeaponStats Health="10" slots="7" type="Ballistic" projectileclass="bullet" numFiring="1" damage="15" heatdamage="0" impulse="0.05" heat="1.0" cooldown="5.0" ammoType="GaussAmmo" ammoPerShot="1" tons="15" duration="0.0" lifetime="10" speed="2000" volleydelay="0" groupedlocally="1" ExplodeChance="0.9" InternalExplosionDmg="20" maxDepth="10.0" critDamMult="0.5" critChanceIncrease="0.17,-1.0,-1.0" chargeTime="0.75" chargeFizzleTime="2.0"/>
Clan Gauss
<WeaponStats Health="5" slots="6" type="Ballistic" projectileclass="bullet" numFiring="1" damage="15" heatdamage="0" impulse="0.05" heat="1.0" cooldown="5.0" ammoType="ClanGaussAmmo" ammoPerShot="1" tons="12" duration="0.0" lifetime="10" speed="2000" volleydelay="0" groupedlocally="1" ExplodeChance="1.0" InternalExplosionDmg="20" maxDepth="10.0" critDamMult="0.5" critChanceIncrease="0.17,-1.0,-1.0" chargeTime="0.75" chargeFizzleTime="2.0"/>
So, the only real difference is that one is guaranteed to explode while always having CASE equipped, and the other one only rarely doesn't explode (10% chance to not explode), while having a 3 ton +1 crit difference (3 tons mostly consumed by just ammo alone). Why does Clan Gauss have all the beneficial advantages again?
A suggestion is to slow down Clan Gauss projectile speeds (to like 1600 or so) and/or improve the Cooldown of IS Gauss. Remember though Gauss is not a DPS weapon... as people try to claim Gauss is some sort of DPS weapon despite having a charge to fire the weapon.
TL;DR
IS LBX needs better spread across the board based on SRM/CSRM spread balance (SRM2/SRM4/CSRM2/CSRM4 too fragile, especially with Artemis).
LBX2/CLBX2 as slow as ERPPC/CERPPC (IS ERPPC needs velocity buff).
LBX10/CLBX10 is pure copypasta and needs to be changed.
IS LBX20 (and Heavy Gauss) needs a massive buff to justify use over LFE+AC20.
IS Gauss still inferior to Clan Gauss (Clan Gauss needs velocity nerf or more IS Gauss buffs).
If left unchanged, IS LBX will generally stay inferior to Clan LBX.