Well I've driven a few of them. Both the regular and the GTP versions of the car (no I've never owned one...or ever will either). However you notice a SIGNIFICANT increase in power/torque in the GTP. It may only make 40hp more, however that is a big increase in some cars.
For example, I also own a 97 Sunfire SE (2.2L uggg only 115hp). My friend liked my car so much he decided to buy a 98 GT (2.4 only 150hp). Both car's aren't exactly the fastest things on the road, however that extra 35hp make a HUGE difference in his car. The GT's actually weigh (with extra's) approx 250lbs more than the SE's (base...which mine pretty much is)
Anyways, my best run in that car was a 16.9s runnign consistant 17.1s times. My friend however, making 35hp more in a 250lbs heavier car, runs a consistant 16.1s with a best of 15.85s.
May seem strange, however difference torque/hp peaks makes a big difference on some cars. This is probably the case with the GTP.
Ryan