Note, you still benefit from those attributed to "Laser" (for your laser, regardless of which kind) and "energy" (for any energy weapon including flamers and PPCs).
So while the weapon specific one will stack with those for double the benefit, you still get generic benefits too.
Quick example, Awesome 8R. Heat generation quirk.
ENERGY HEAT GENERATION: -12.50 %
ER LARGE LASER HEAT GENERATION: -12.50 %
The two stack, allowing you -25% heat for your ER Large Laser.
ERLL heat in MWO, base value, is 8.00 heat (12 heat in Battletech).
Now... 8.00 heat minus 25% = 6 heat.
Comparison!
Large Laser heat in MWO base value, is 7.00 heat. (8 heat in Battletech).
Not an ER LL, only energy heat gen is applicable. -12.5%
7 heat - 12.5% = 6.125 heat for the LL.
Since they have identical tonnage, slots, etc. for the IS mech,
you have Zero reason to use an LL over an ER LL. Though you could to almost the same effectiveness.
EDIT!
Actually you do. That's right, PGI did something slightly smart in changing the duration of the ER LL compared to the LL in base stats:
The ER LL has a default duration of 1.25 seconds.
The LL has a default duration of 1.00 seconds.
The Awesome 8R used in this example has no laser duration modifier.
But there's still another factor here. Cooldown time.
Each has a cooldown time of 3.25 seconds after the beam ends.
ER LL 1.25 second beam time + 3.25 second cooldown = 4.5 seconds from first shot to second shot.
LL 1.00 second beam time + 3.25 second cooldown = 4.25 seconds from first shot to second shot.
By default LL shoots faster.
Now... The Awesome 8R also has a cooldown bonus for energy weapons an a stacking ER LL cooldown bonus.
It is again -12.5% (hardly original).
ER LL + energy cooldowns = -25%.
Revisiting the above dilemma...
ER LL = 3.25 - 25% = 2.4375 + 1.25 beam duration = 3.6875 seconds from first shot begins to second shot begins.
LL = 3.25 - 12.5% = 2.84375 + 1.00 beam duration= 3.84375 seconds from first shot to second shot.
---Oh. N/m. Absolutely no reason to use an LL over an ER LL on the Awesome 8R in the long term. But it comes super close. The only reason, at all, to use the LL is for the shorter beam time which remains 0.25 seconds shorter.
In some cases, you'll still get superior results even if you don't use the chosen 'double' quirk weapon.
In the instance of the Awesome 8R, this is barely negligible.
Edited by Koniving, 01 January 2015 - 10:09 AM.