At the local track here, there are a few of those GTP's in the 13's. There is one that is running in the 12's.
I think with only an exhaust and different pulley on the supercharger and he'll will most likely see high 13's.
You should take him on the street if your on your toes. Just because he ran a low 14 on the track doesn't mean he can do it on the street. Your WRX should run the same on the street as it does at the track due to the AWD. You might even be slightly faster on the street because there is no traction compound.