Unless your auto tranny is toast, and you absolutely MUST have a 5/6 speed, for me, the way to upgrade the car would be a small supercharger giving about 100hp, and a beefed up auto from the likes of Level 10.
The present tranny can handle the load, but not the heat. the engine is so high on torque output, that even as N/A, the tranny is making a lot of heat. So add a cooler to save it.
This implies the transmission needs to be upgraded for torque, the horsepower is incidental. The power curve of the engine, even with or particularly with a charge cooled and driven supercharger, that the manual stickshift is really only for street cred, will add very little to what the car can do.
Graham in the UK does not agree with me on this, he reckons the manual box is about 200Kgs lighter than the auto. I agree and know it would be lighter, but question 200Kgs, and would like to see both cars on the scales to verify.
Ever check the weight of a WRX?? Not far off an SVX, even with manual.
Joe