Wait a minuite.. Your turbo takes too long to spool? How long are we talking. It's only an 18g if I remember right. They're not massive turbos. You should be seeing boost around 3Krpm. If you're going to upgrade to anything, it's going to spool even slower.

As for an upgrade, with out having to replace everything in the world, you can bump that thing up to a 20G and be capable of near 400whp. The 18G should be sufficient to around 350whp.