While I have limited track time in my GT, I still think I can comment regarding the overall performance of the car vs other high perf contemporaries. The first time I took the GT to Autobahn in Joliet, the track was shared with the local Porsche club. My on board instructor who races an SCCA BMW M3 was really impressed with the GT abilities, particularly on the 1400 ft straightaway. After 6 20 minute runs of the track, I had really improved my times, and could basically run down nearly anyone ON THE STRAIGHTAWAY. This was not wheel to wheel racing; however, with the instructor, I was really able to keep pace on more experienced cars/drivers because of the GT's abilities. My car is stock the than the X pipe. With a pulley, the car's acceleration would be even zanier.
I think the issue for many Porschephiles is the size of the GT; certainly most courses are too small for it's width and capabilities. Additionally, the GT has stayed relatively expensive, but after a year or two, only the rarest of Porsches haven't plummeted in price.
My advice is if you can afford one, go for it. Much more visceral than the Italians, and far greater soul and passion than the German cars.