Your stock turbos can gear near that hp range, i've seen 400+ whp on stock turbos from various members. I believe the GT500 and the Sport 500's are pretty much the same but I could be mistaken. If you're going to upgrade turbos, go with JWT 530bb's or some custom made GT2560R turbos. The custom made GT2560R turbos you can make yourself for about the same as the Sport 500's and have 500whp potential, faster spooling and better longevity.