Bluebeast pretty much hit it on the head with the reliability. To get that particular 6 cylinder motor to 500 rwhp, u would have to do major head work, upgrade all internals to handle the stress, and have some high boost numbers. by the time u re-vamped ur engine so extensively, u might as well have put that money into a V8 and still had money left over for mods, and added reliability.
2005 Mineral Gray GT:
GT500 Front, Cervini Cowl Hood, 93 Oct Bama, K&N CAI, Forged pistons, Forged rods, Stage 2 VSR Comp Cams, JBA LT headers, O/R H pipe, MAC polished SS Mufflers, Corbeau strut tower brace, AL driveshaft, Tokico Struts, SR lowering springs, Steeda anti-roll bar, BMR adj. upper & lower CAs & panhard bar, 4.10 FR gears, 18" Foose Nitrous wheels, Roush spoiler