I think it actually makes sense talking about DPS, especially regarding the different kinds of ACs...
The RACs for example do only minimal damage with a single shot, but have a high fire rate and in fact a high DPS, while LBX20 has a very high damage per shot but an extremely long reaload time, which makes it's DPS rather low.
Standard ACs/UACs are something between those two.
Due to these differences each weapon has it's specific role, while RACs are good for long firefights, LBXs are most suitable for hit'n'run tactics.
Considering that, if LBX where brought closer to ACs (in the way they work) one of both weapons would become more or less useless.
Regarding the LBX20s ability to oneshot ASF, I don't really see whats wrong with that.
I'm pretty sure a shiva takes more then one hit.
However, even if it doesn't, it's still pretty hard to hit an ASF with an LBX, and besides the LBX20s range is only about 350m, effective vs ASF at about 200m I'd say.
And in my eyes getting that close to an LBX20 equipped mech/vehicle while in an ASF is stupid, especially because you simply don't need to get that close.
I've already been that stupid several times, and it feels just right that such stupidity gets punished...

Just my two cents.