better yet, what type of internals do you have
Assuming stock pistons I personally would not try to push it much past the 350-370 whp mark. Granted, at 25% drivetrain loss this puts the power at 440-470 hp. The next size increase in injectors is: ID 2000 or FIC 2150. Alternatively, you could try increasing fuel pressure, but that would require a FPR. Either way, it looks like money will need to be spent if those power levels are desired.
As a heads up on the FMIC, if you live in a hot climate I would suggest investing in a upgraded radiator since the airflow to the stock radiator will be greatly decreased with the FMIC. This was only an issue for our customer in Arizona where the temps are 90-105 in the summer.