Actually the Priest pretty much sucks. I've been paying close attention, and the reason you don't get much XP is because the gun does minimal damage at best. I notice that it hits large tanks with dead on shots for like 7 damage, it will take out the smaller tanks fairly well but once you get into Tier 7 battles it sucks giant donkey balls and you damage drops to the lowest of any of the vehicles i've played so far in this game.
-Hippie