[TurboGT]

Don't ever boost more than a couple psi in 5th and 6th unless you're well over 100mph. Do a search for LSPi, you'll knock a piston out of these engines very quickly.

Cobb tunes are very aggressive and also make it very easy to kill your engine on these cars. Don't run them.

Pick up an Intercooler. Then get an adapt-x or custom tune.

Intake gains nothing, catback gains nothing, downpipe is very small, un-noticeable gains. Intercooler is your meal ticket.

 

[TurboGT]

Don't bother with piping either. I have big turbo with stock piping, it isn't a limiting factor unlike people would lead you to believe. Just trying to save you some coin!

Yes it's fine to run stage 0 with those mods.