Somebody found a bug in the plasma rifle code that kind of nerfs it in single player. I suspect that the major mods have fixed it, because I played Revision and GMDX last year and the plasma rifle seemed more effective than what I remembered. It's still not powerful enough to justify the inventory space it requires, but it's an OK middle distance weapon in a heavy-only build like yours. With master heavy weapons skill you can even snipe with it against stationary targets, to save rockets. I usually don't bother with it because it's just boring to use. Especially when your other weapons are the flamethrower and GEP.