I'm in the process of locating a 2007 GT to purchase. Looking towards the future, I want to add a supercharger. What power levels can the automatics and manuals take? What mods are needed for the setup to handle 400-500 HP range?
Many guys including me have been just fine with the manny tranny at 500 rwhp. They hold up pretty well. You'd need an aftermarket clutch package at that level though. I also suggest Prothane motor mounts and the new MGW shifter if you plan on getting a good speed-shift down. The autos should be kept at or below 400rwhp for sure. There's a new auto tranny upgrade kit available now that will easily handle the 500rwhp. You can't use aluminum driveshaft on the autos because of a vibration that nobody seems to be able to stop even though the autos and mannys share the same shaft and crossmember.
I have had an aluminum driveshaft in for 8500 miles without a vibration so that statement is not correct. (Mine is a Powerhouse) However some driveshaft makers have experienced problems with vibrations in automatics. I have tested mine to 165 MPH and it performs perfectly.
The automatic transmissions are failing pretty frequently with forced induction. Third gear is the weakest point in the automatic tranny. The torque converters are a weak point also.
My third gear died after 3500 miles at 10 PSI. I have the upgraded Level 10 Stage III automatic transmission and it has held up to 11 PSI abuse without a hitch. Level 10 also sells the upgraded bands, clutches, pump and solenoid in a kit if you like rebuilding trannies on your own.
You are a lucky guy because I know 2 guys that had Powerhouse aluminum shafts that vibrated badly with autos. One of them is a stangnet member who went through 3 different brands before giving up on it. It's more likely than not that they will vibrate.