Overall, rapid fire weapons have an excellent DPS. The only drawback of a plasma gun is that you can't store enough ammo.
-MAc solves this problem. With the nuclear plasma rifle from the spider's lair (hopefully, now removed), you'd never be short of ammo. unlimited ammo (and moddable) plasma gun is ridiculously overpowered. I guess it will be (just a little) harder now. (I think this still needs some nerf)
-MCe can lead to great damages too, but it's not really a "rapid fire" mastery, it's a "sniper" tool. Killing monsters before they get a chance to see you is always great, and works not only with rapid fire, but also with guns or RL (and even nuclear BFGs :p). from my point of view, MAc is more powerful, but MCe is the lamest mastery
-I didn't even try MEn yet... looks useless to me since it doesn't work when not chain-firing. Numbers seem good, but too constraining, imo.