Haven't driven the GT 500 but I did drive a Roush. We were shooting a race themed video in Las Vegas, a Viper vs. a Roush (OK, not a reality show!). The Roush felt just great and the sound was fantastic. I've been thinking about it ever since, a great top-down hot rod to head out in the hills when I have to get away from my office.
Didn't get to really wind it out, though, and I suspect in the end I'd rather have a Boxster S when you were tired of just cruising and wanted to really get on it.
D.