Considering the A500 is basically a 904 with overdrive (which is the same on the A500 and A518) I think it'll take quite a bit as long as it's in good shape or properly rebuilt. Guys regularry run 904's in 3200 lb. cars running 10's so their a lot tougher than we used to think. The o.d is the weak link I've read but I've also read that it'll take 400+ hp without any mods. Not positive on the upper limits but I believe SMR advertises a A518 that's good for 1000 hp.