we have a GT-RS on our car and it is a nice bit of kit. slightly more power everywhere than a GCG hi-flow (but it is slightly more expensive). if money is no object the HKS 2835ProS would be pretty nice with a tiny response penalty but higher flow available on both sides of the turbo (ie more efficient).
i wouldn't want anything much larger on a street stagea due to the extra weight you need to lug around.
a GT30 specced correctly (or the more exxy but similar HKS 3037 ProS) with 260 deg tomei poncams to bring the boost threshold down would be ok but there is a little bit more work and time fitting a GT30.
think carefully, as it is an expensive exercise buying one that doesn't suit what you want, however, don't try and do it on the cheap.
...and don't get suckered into bigger is better cos it just aint so.