Stat wise...they don't seem too different from the heavier things, same Z thrust, same forward boost across all classes. Perhaps it's just the weight of the mech itself in the calculation that makes the difference?
JJ stats:
-<Module CType="CJumpJetStats" name="JumpJets_ClassI" id="1500"> <ModuleStats components="centre_torso, right_torso, left_torso, right_leg, left_leg" health="10" tons="2" slots="1"/> <Loc iconTag="StoreIcons\JumpJets_ClassI.dds" descTag="@JumpJets_ClassI_desc" nameTag="@JumpJets_ClassI"/> <JumpJetStats maxTons="200" minTons="90" heat="3.0,0.09" turn="0.7" boost_z="68" boost_fwd="4.1" boost_instant="220" duration="4.1" cooldown="0.53125"/>
-<Module CType="CJumpJetStats" name="JumpJets_ClassII" id="1501"> <ModuleStats components="centre_torso, right_torso, left_torso, right_leg, left_leg" health="10" tons="1" slots="1"/> <Loc iconTag="StoreIcons\JumpJets_ClassII.dds" descTag="@JumpJets_ClassII_desc" nameTag="@JumpJets_ClassII"/> <JumpJetStats maxTons="90" minTons="80" heat="2.25,0.25" turn="0.6" boost_z="68" boost_fwd="4.1" boost_instant="200" duration="3.75" cooldown="0.5"/>
-<Module CType="CJumpJetStats" name="JumpJets_ClassIII" id="1502"> <ModuleStats components="centre_torso, right_torso, left_torso, right_leg, left_leg" health="10" tons="1" slots="1"/> <Loc iconTag="StoreIcons\JumpJets_ClassIII.dds" descTag="@JumpJets_ClassIII_desc" nameTag="@JumpJets_ClassIII"/> <JumpJetStats maxTons="80" minTons="60" heat="1.7,0.09" turn="0.8" boost_z="75" boost_fwd="4.1" boost_instant="200" duration="3.75" cooldown="0.5547"/>
-<Module CType="CJumpJetStats" name="JumpJets_ClassIV" id="1503"> <ModuleStats components="centre_torso, right_torso, left_torso, right_leg, left_leg" health="10" tons="0.5" slots="1"/> <Loc iconTag="StoreIcons\JumpJets_ClassIV.dds" descTag="@JumpJets_ClassIV_desc" nameTag="@JumpJets_ClassIV"/> <JumpJetStats maxTons="60" minTons="40" heat="2.0,0.06" turn="0.9" boost_z="75" boost_fwd="4.1" boost_instant="120" duration="3.75" cooldown="0.7"/>
-<Module CType="CJumpJetStats" name="JumpJets_ClassV" id="1504"> <ModuleStats components="centre_torso, right_torso, left_torso, right_leg, left_leg" health="10" tons="0.5" slots="1"/> <Loc iconTag="StoreIcons\JumpJets_ClassV.dds" descTag="@JumpJets_ClassV_desc" nameTag="@JumpJets_ClassV"/> <JumpJetStats maxTons="40" minTons="20" heat="1.7,0.09" turn="1.0" boost_z="37" boost_fwd="4.1" boost_instant="110" duration="3.75" cooldown="1.0"/>
Or perhaps a more readable form:

Unknowns are the Heat, and Cooldown at a glance doesn't look right. I'll assume Cooldown is a penalty to the stock time, whatever it is, meaning Assault JJs take nearly twice as long as Light JJs to recycle...if that's right.
In my testing, ~10 seconds for a Cheetah for max JJ burn to full recharge, and ~13 seconds for a Victor, both having 6 JJs.
Math doesn't work out in that case.
Cheetah max height was 66 units vertical
Victor max height was 43 units vertical
Both having 6 JJs, though the Cheetah pays 3 tons to the 6 of the Victor.
That would be Class 5 and Class 2 JJs (Victor being the only mech thus far with Class 2s, 80 and 85 tonners)
3JJs on a Spider gets you 37 Z height
3 JJs on an HGN gets you 26 Z height
Keep in mind, the Spider pays 1.5 tons for that, to the 6 tons of the HGN.
Now, I can understand wanting to keep Class I JJs down, as poptarting Whales is a fearsome consideration. However, at the moment, the cost to performance ratio of JJs is pretty bad.
Unless forced (see Omnimechs), it seems most people do not take a full compliment. One is all that's needed to traverse pebbles or bunny hop your way up a sharp incline, and they spend the tonnage on heatsinks or weapons.
One Jump Jet gets you nearly all the benefit, while additional ones seem to have not much effect.
Admittedly, my Hunch IIC does have a full JJ compliment, as I've made it a poptart. The 4 JJs net me ~50 units vertical height, which is plenty with the high mounts and general effectiveness of Class 4 JJs (the best of the bunch).
A single JJ nets you 17 units.
2 JJs nets you 28 units
3JJs nets you 39 units
Then the full compliment ~50
Noticeably linear, roughly +11 units per JJ, after the initial 17 units.
Compared to the Class Is on a HGN
1 nets you ~14 units
2 nets you ~19 units
3 nets you ~26 units
A similar linear amount, but this time only 7 units.
This costs 6 tons on the HGN, to the 2 tons of the Hunch, for significantly worse performance. Albeit, relative cost for the 50 ton mech to 90 ton mech makes it ~4 tons for the Hunch, still an improvement. (unscientific calculation of total mass, ignoring engines and stuff)
Would it be better if more JJs gave you a noticeable improvement? More of an exponential curve to the current linear?
The current mediocre 1JJ stays, but taking a full compliment adds a not insignificant improvement? Enough thrust or height that an Assault can actually use their 6+ tons of investment to gain both decent height, while also not significantly damaging their legs, maintaining enough fuel to slow descent?
I'd almost suggest a bonus given when a full compliment is taken, but mechs with a low cap, like 2 of the Jester/Huggin (not a bad thing) or 3 of a Whale, are given a fair advantage, while high caps like 10 of the Spiders, Jenner IICs or 5 for the Heavy Metal are hurt.
Alternatively, just buff the other JJs to the tune of the Class 4s in effectiveness. Give Lights more Z thrust, as they have a hard time clearing buildings without humping them first. Reduce the duration of Assault JJs (at least...I think that would make the Thrust happen faster, same thrust, less time), to make them less HoverJets™, but make a significant movement.
If HoverJets™ are transitioned to Jump Jets, cooldowns could be touched to prevent excessive spam on the heavier end, while Lights and Meds should probably retain roughly current values. Both for the mobility, and I daresay for the Med Poptarts (of which there are few, on both factions).
There is the possibility of poptarting returning...but that's not a bad thing in itself. The issue back then was EVERYTHING sucking balls in comparison, and Poptarting weapons being the best overall thing on the battlefield. 40% damage nerfs to SRMs, Lasers with wonky hitreg and poor stats, no quirks.
The game is in a very different state, where SRMs only suck relative balls to lasers, and will beat PPCs in a brawl (while back then, they were more akin to spitballs, and PPCs brawled adequately, and (u)AC5s wonderfully).
As it stands, certain Meds can be adequate poptarts, the BJ-3, Nova and Hunch IIC (I find acceptable, Gauss+PPC) among them.
Those are, at most, 20-25 PP FLD, which pales in comparison to 45 at the peak, or the 30-35 that followed it.
Compare that to the Laser or SRM alphas? SRM24 is 48 damage, a pack of MLs 30, Tri of LPLs 33...mix and match those, and you exceed the PP FLD, with higher DPS and sustainability on top of it all.
In other words...Poptarts wouldn't be top of EVERY foodchain, but have a place, good in a certain niche, but will lose in others.
As it should be with balance and weapon types.
TL:DR
HoverJets™ aren't great. Meds can jump alright. Opportunity Cost for JJs are kinda rubbish in most cases.
- Normalize JJs around that performance mark?
- Make them more powerful, but shorter duration? Jump instead of Hover?
- Give JJs an exponential curve of effectiveness instead of Linear?
If none of those look appealing, what would you suggest for helping the JJs? Disclaimer: If you say they're fine, I might have to slap you.
If someone wanted to get some extra number comparisons for me, that'd also be nice. How about 5JJs from each class? I cannot do Assaults Class I (Heavy Metal), but should have most other Classes (Summoner, Wolverine, large selection of Lights).