Well the gt28r will net you ~400hp tops.. pending the hotside, boost level, and fuel used. I think a realistic goal would be 75-100 hp over what you have now.
He WILL need to do rods and pistons to get there thu. This would also include injectors, pump, FPR, and I would seriously look into an EBCS (electic boost control solenoid)to keep the spikes down. Also the stock ecu may not be able to handle this load, so a stand-alone unit may required.
To stretch out to near 500hp he will need a minimum of a GT35r or a pte 6262, or comparable. I am not trying to rain on your parade, but your parts list and your goals are not in line. Wish you luck thu..
Edit:... Looking at the Mazda3 forums, it looks like his goal with the parts listed are closer to 200-275hp range. The Trans looks to top out for reliability ~275hp without some attention to 3rd and 4th gear.