I always thought the 427R came with the Eaton M112 like the 03-04 Cobras. The M90 seems small with a 4.6. Are there any gains with porting the M90 like the M112 was able to get?
In my opinion, the Roush is the ultimate bolt on GT. The Roush seems to be a great choice if you're into road racing. The engineers didn't overload the internals staying at 350-370rwhp and the upgraded suspension can put it to the ground. Haven't seen stats but I'm guessing high 12's to low 13's in the quarter at traps of around 110, which is pretty fast so you can run with the 6.2 camaros and 6.1 challengers in the 1/4, yet outrun them in the corners.
For the price though, arent' the GT500's and 427R's close to the same in price? The GT500 might be 300lbs heavier, but with some practice it should be able to stay with the 427R on a road course. The GT500 is the ultimate Mustang. Forged 5.4 with a larger capacity SC that has room/volume to put out a lot more with a tune a pulley change than the M90 in the Roush is capable of. I've driven the previous generation GT500's and 427R's and yes the 427R feels more settled and lighter, yet the GT500 seems like it can take day to day romps on the throttle easier. The 427R, although very smooth, feels complete/maxed out where the GT500 is holding back if that makes any sense. I personally don't road race or drag race, but I do appreciate low end torque. Money not a factor, I'd get a GT500.