If your trans is already feeling weak then it is not going to live long after adding more power, but you could drive on it after the top swap is done-it is just going to fail sooner than if not. Internally there is nothing special between a GT and GTP trans other than for the supercharged transmissions they recieved a slightly larger torque converter and a beefier differential which are both things that can be added to your existing trans. Installing a shift kit may prolong the life of the trans but again if it is already headed down the road to failure it is only going to be a matter of time.
