It's a full 5 minute WEP like most planes. Only plane with less than 5 minutes is (if I recall) the Ki-84, with 90 seconds or some freakishly short time.
The plane is big, yes, but the WEP boost is significant. It's also a clean airframe that reaches high speeds easily. Don't count out that WEP setting as inconsequential. It makes as much as a difference as it does on, say, the spit, or the p47.
EDIT: P.S. I agree with the "bad gunnery" comments. You should seriously have had 10 kills with your position and ammo loadout. You dove in behind them, all tail astern. You could have just latched on to tail end Charlie and worked your way to the front racking up tons. Only takes a 1-2 second burst to kill a mossie (or disable it) from 6x50cal.
