I think going GT-RS is a bad decision. If you need to upgrade turbos, get -5s at the maximum. I think Piggaz will no doubt give you a nicer insight if he see's this thread as even with 2.8l, his GT-RS setup is extremely laggy.
-5s are noticeably laggier than GT-SS (-9), but will make more power.
I've just done a similar swap from -9 to -5... Personally I'd not do it again given a second chance as just general driving around the response was amazing.
That said -5s certainly come into their own above 150km/h but forget it below that as you have to ring the cars neck to makeup the difference.
If you go E85 on the -9/GT-SS's you should be able to make around 360rwkw, maybe 370rwkw which is what -5s do on PULP.
This would certainly be the best of both worlds where you get response (and added response again with E85) and the higher top end.