I think a stud girdle would increase how far you could push it. Once I get the cage and stuff in mine, I plan on juicing it with a 100 or 150 shot, which will push it well above 600HP. I only spin mine a little past 6K.
I never used a stud girdle and put in 400hp on 2 stgs, i have broke cranks, pushed out gaskets, but not the block using n2o, but i leave off idle on the system and that seems to play mayhem on the bearings which will make them need changing more often.