I think your goals/restrictions will play a factor in your outcome...
Do you need to pass smog/emissions?
Will you be driving your car on the street?
A good heads/cam, full exhaust package can bring you safely into the 550-575 rwhp range.
Just for reference: My 2001 ACR (stock) dyno'd at 398 rwhp and 442 rwtq...
First, I installed the LETHAL 500 package (full exhaust, filters) which brought power up to 431 rwhp and 483 rwtq...
Then came the LETHAL 600 package (heads/rockers) which brought the power up to 520rwp/540rwtq. You may be able to reach your power goals with some heads worked properly.
Then came the LETHAL 650 package (cam, TBs, airbox) which increased the power to 572rwhp/579rwtq
The LETHAL 750 package to me up to 650+rwhp and 700+rwtq.
The LETHAL 1000 package is taking up to xxxxhp and xxxxtq (still testing).
Are you asking about MPG (miles per gallon) or did you mean MPH (miles per hour, ie 1/4mile).