Hands-down, an ER-MLas will always be better than an AC-5, an LRM-5 superior to an AC-2, and for the cost, a pair of 2 regular PPCs out-perform an AC-20. There are too many penalties on autocannons. They're big, heavy, still cause massive heat, and need ammo. The only perk is that they are cheap, and as an in-game effect, that doesn't matter.
I like some of the ideas you folks came up with, but in my eyes, there is a real easy fix to the Autocannon problem (aside from just getting rid of them).
Either make them lighter, and/or make them more compact, and give them all the same range. Autocannons should fill the niche between LRM and SRM, because lasers and PPCs cover everything else.
AC/2 - 2T / 1 Crit / 1 Heat - 1/5/10/15 Range
AC/5 - 4T / 2 Crit / 2 Heat - 1/5/10/15 Range
AC/10 - 7T / 3 Crit / 3 Heat - 1/5/10/15 Range
AC/20 - 12T / 5 Crit / 5 Heat - 1/5/10/15 Range
Using the same ammo values, those stats would fit nicely between LRM and SRM. And both L.Las and PPC fit well with them too. For the same damage potential and slightly better range, you'd take a PPC over an AC/10, but pay for it in heat. Both LRMs and SRMs have slightly better weight/crit costs, but the AC's trade-off is doing full damage with each hit, instead of a chance to do a fraction of what's possible.
That's my $0.03 on the matter.
Oh, and Ultra/Rotary/LB... That's a whole other can of worms I ain't touching.