I know of a few buddies of mine who have BPU Supra's still on stock twins that put down 420 whp. So that number is not far off at all. However, something is messed up with that pull. Did you reset the timing on it before doing the pull? Supra's are weird in that if they ever sense knock the ECU will automatically pull timing to prevent any damage, hence lower hp. If you want, I can give you the guys SN and I'm sure he can help you find out where that hp went.