Sky brings up a good point: Why not just put on a smaller pulley to get more boost? As I'm sure you know, the D-1SC is a serious piece that can produce 15 psi without breaking a sweat!
As for the flywheel, you may want to look into an aluminum unit (mine weighs 11 lbs. vs. my old stocker which weighed in around 25 lbs!) The driveshaft will net you around 6 lbs. This will help your HP numbers for sure.
Good luck getting 500 - I think you'll be very close. Keep us posted.
E