we have been doing 450-500whp on a stock cam/rockers with just some valvespring upgrades out this way.

If you get a stattama kit, you pretty much have to stick with a garrett style T4 turbo to make it worth even paying for a kit. There are plenty of great turbos out there used (check out the turbobuick.com forums). A GT35r in T4 trim is probably hard to come by for a reasonable price. I like to get journal bearing turbos anyway...

I would grab something in the 62-67mm range.