Actually I totally forgot about damage against inner structure and forcing PSRs. Those factored, theres no reason to use REML over ML except maybe against special armor BA and possibly vehicles(dont care about PSRs, little to no inner structure).
LL, SNPPC and PR against RELL: yeah they lose a bit of efficiency against special armors, but they are still better choices most of the time. LL loses by 36% which but contributes more towards PSRs and chews IS better(weight and heat need to be factored). Also sinks > gun mass.
ERSL is just useless. Even standard SL is just as inefficient against the special armors and it wins by 50% towards PSRs and against inner structure.
But when comparing guns with each other there are often range profile differences and ammo and so forth, so things can become a bit fuzzy. To make it simplier, lets look at weapons that need no ammo and share the same ranges. Out of those weapons, a base to-hit of 8 is enough for X-medium-pulse, a gun not considered very good at all by most people I know to have an opinion on it, to be more efficient anti-hardened armor and such gun than the REML. Let me demonstrate:
To-hit odds difference between REML and XMPL with base to-hit of 8(REML needs 8, XMPL needs 6)
(26/36)/(15/36)=1,7333....
Allocated weight, how much damage a turn you get by allocating 1 ton, against the special armors while using DHS(notice the weapon ranges are same)
REML: 6/(2,5+3,5)=1,0
XMPL: 3/(2+3)=0,6
1/0,6 = 1+(2/3) = 1,666.... < 1,733...
--> XMPL is more efficient when base to-hit number is 8 or higher. Using 4/5 pilots, that is basically almost all of the time, and a lot of the time even if the firing range is 3 or lower and in the short range bracket. Only advantage REML retains is double the hit cluster size(which isnt meaningless, and can possibly make REML+TC combo better for aerospace fighters perhaps?), but it loses 20% in damage counted towards PSRs and another 20% against damage to inner structure which is not a small thing when facing meks. On the top of it all, a larger portion of REML's required weight consists of the weapon itself: by using XMPLs you get proportionally more heat sinks that can be used for cooling other guns or systems.
Medium and small VSPL dont share quite the same ranges but they are both even more efficient at ranges 1 and 2 thanks for huge that to-hit odds bonus. Medium VSPL also happens to have exactly the same BV value, 56, as the REML.
But what about REML+TC combination against XMPL?
ERML: 6/(2,5+2,5/4+3,5)=0,9056603774
XMPL: 3/(2+3+0,5(for TC))=0,454545...
ERML/XMPL: again the same 1,66 --> What happens is the to-hit required for XMPL to be better is a step higher at 9: (26/36)/(15+36)=1,733... Sorry about the maths. ;)
My conclusion: I believe that for usual ground battles at least the XMPL is, facing special armors or not, a better choice than the REML for all 4/6 or faster meks and 5/8 vehicles as well as jumpers, as they can generate 2 or more movement modifiers consistently and can control engagement range at least somewhat. For the slowest of units there are probably more requirements for their close-range backup weaponry than being slightly more efficient against specialty armors than more conventional options.
What do you people think about the RE-lasers on Aerospace units, intended as a counter against possible laser-reflective armor units? Do standard energy weapons' full damage count towards structural integrity treshold? If not, I could see the RE lasers have a use right there.
edit: fixed typo