Honestly, I think it's more fun even stock. Yes the GT has all that power, the V8 growl, and you can hang the ass out all day long (I miss my IS300 for that) but you can feel it's size and extra weight. I am convinced that I am happier with the FiST than I would have been with the Mustang. I don't need gobs of power, just something fun to toss around.
This gen Mustang is leaps and bounds better in the fun department than any other gen mustang ever built though.