Even then. Heavy with a custom gatling gun vs a medium with a custom assault rifle..sure the heavy will win..if he can keep the medium in his sights long enough. But if the medium is effectively using SMART...it should still be pretty even.
If I ever see a heavy with a minigun I will not engage in a straight up firefight. A big gun does you nothing if you don't have time to shoot your target.
I Can see how someone can boost, their is Co-op so all you need is a medic and person who is willing to absorb a lot of damage while the medic buffs him, or in a Multiplayer match, just the same except now 1 person can shoot the other for XP but don't kill each other since each shot gives you XP
Shooting your teammate results in negative XP.