I looked at the Plasma Cannon this last pass on the rebal mod.
It's basically falling prey to the idea that if A is balanced, 2 * A is also balanced, without considering the real cost of forced burst lengths (that isn't a positive).
It's actually a pretty lousy weapon when we factor in that it's going to use up 3600 Flux whether or not the last 2400 is wasted damage. Misses, overkill, etc. etc., and all at a range that isn't a huge step up- 100 range is just not that important of a difference in a lot of situations where you have a ship that can actually mount Plasma Cannons in the first place and you're not really able to kite much. So yeah, it's great for alpha... if you hit, and if none of the shots are overkill. Otherwise, no.
The weapon could have the burst length reduced to 1, which would largely fix it, stat-wise, but that's pretty boring. If low range and bursts is the theme, then buffing the base DPS is the best way to get it right. I am trying about 1:1 in the current build of the rebal mod, which puts it on par with the pew-pews; the cycle time between bursts, and the huge burst cost, probably compensate.