We're never going to get XL parity. I'd suggest the following instead:
cXL is in a fairly healthy space (without considering IS engines). It's our baseline.
IS XL: Increased speed by 7.5%/10%/12.5%/15% for lights/mediums/heavies/assaults, applied additively alongside Speed Tweak (that is, full speed with both IS XL and Speed Tweak nodes would be 115%/117.5%/&c.). That'd put Locusts up to 177kph (from 165) on one end of the spectrum, and Assaults up into the mid 70s on the other-noticeable, and maybe worth risking, but still a tradeoff given that the torso pop is still a thing.
LFE: Equalize ST loss penalty at the cXL level, add heat dissipation buff of 10%/7.5%/5%/2.5% (balance aside, the logic being that smaller 'mechs have a greater surface area to volume ratio, i.e. more efficient heat dissipation). IS 'mechs still often have engine caps below what's carried by equivalent-tonnage omnimechs and have lower tonnage savings than cXL when taking LFE, but the equal loss penalty and cooling buff help make things a bit more even.
STD: Substantial structure buffs for all torsos. Heat dissipation buff of 15%/12.5%/10%/7.5%. STD 'mechs are good at one thing: energizer bunny mode. This makes them live longer and keep firing longer (which helps make up for their generally lighter armament-lower alpha still, but better staying power in fights).
The idea's that cXL is the JOAT, it's going to be solid at everything, and PGI won't give the IS an equivalent, so the three engines available to the IS should instead outperform cXL in one area to make up for their sharp downsides elsewhere. IS XL gets better speed but still has much less effective cooling and dies when a ST pops. LFE has the same survivability and somewhat improved cooling (relative to IS XL), but still weighs way more. STD has very good cooling and survivability, but weighs so damn much. And yes, clan Battlemechs that swap over to standard engines should get the same structure and heat dissipation buffs.
Just spitballing on numbers there, and even on effects, it's more about the general concept.
Edited by Alexander of Macedon, 05 August 2017 - 07:11 PM.