Honestly I don't even know why the high-bore ACs even have splash. The entire balancing aspect of autocannons vs. LBX is that the autocannons only hit one thing, so if you miss, you miss. The enormous nuclear blast the AC20 has just invalidates this weakness altogether. If you removed the splash from the AC10/20 entirely, LBXs are now fixed (though it probably still wouldn't hurt to tighten the spread up a bit, just to take it out of breath-mint-range so I don't have to ram my mech into enemies to use it). But I have a feeling that removing the splash is going to be insanely unpopular.
On paper, the LBX is superior to the AC20 in every way. Better heat (well, it's actually the same, but the UAC20 fires two shots instead of one). Better damage (2000 potential if every pellet hits the same spot - honestly I don't think this EVER happens, even at super-close range). 4:3 refire rate advantage over IS autocannons (less so vs. CUAC20). The AC20 has massive splash damage, much better range, and, ultimately, precision.
Maybe you're right, the enormous spread might be hurting them too much. Right now it's like sawn-off shotgun vs. rocket launcher. What I don't want to happen though is the LBX becomes TOO powerful in breath-mint range, which may have to happen. And again, LBX-2 and LBX-5 exist (2 isn't used yet), I can't see any point to these guns at all

Though with the AC2/5 nerf vs. mechs they might end up being much better choices as a result.
This is getting a bit rambly - I'm going to bed with one final note - LBX20s would be freaking awesome if mech knockdown was in-game
