First off, I drive a wrx . If you want a car to make all out power, the STi is the wrong car. It isnt meant to make massive power like a Cobra is. I think that is pretty obvious from just looking at the engine choices. That being said, I think comparing the two cars is a valid thing to do, they cost relatively close to each other, and have similar performance stock v stock. Staying strictly stock, the STi is the winner in performace per $. However when we start talking about modding, the Cobra is the obvious winner, as you can do pretty much anything and everything that you can think of to a Mustang.