I don't know why people keep saying the Stafford GT kit has problems. It is a fully released kit and is a very good kit.
As for the exhaust... I highly recommend doing a custom 3" like a few people have suggested. Because of the lack of Turbo systems out for the Celica in the past, all the exhausts in the market has been designed with natural aspiration in mind. As far as sound is concerned, you can get away with having normally loud exhausts setups because the Turbo will absorb most of the sound created by the combustion process.
If anyone has questions about the SF Turbo systems, please contact me. SF makes awesome Turbo kits for the GT and GTS. Here is my email address.Andrick17@yahoo.com