Both, but obviously it's more of an issue on the track. But even on the street I feel it could use more power. Accelerating onto a freeway ramp or coming out of a corner, in particular. It's a champion in the corners and in transitional handling, I think more power would make it special in every way.
Btw I have a Gt3, not a turbo which is not too far off in power output from the GT4 and might very well be outperformed by it