What are the differences in a 2 seater na exhaust and a turbo exhaust? Are there difference in aftermarket exhaust for the 2 and are they interchangeable?

The factory cat-back exhausts are different between 2+0 NA and TT but aftermarket exhausts are the same for the two (they are interchangeable).

The only relevant difference between systems when going aftermarket is the fact that the NAs don't have pre-cats and therefore have different cats instead, so test pipes and high flow cats will be different for NA and TT.

I have noticed some test pipes for turbos have no cats, but na down pipes do, so they are not interchangeable?

test pipes dont have cats, they replace cats.

test pipes are different between turbo and non turbo. TTs have an exhaust manifold connected to the turbo, connected to the downpipe, and then connect to the TT test pipes. non turbos have headers, which are then connected to the n/a test pipes.

correct....test pipes are not interchangeable, but cat-back exhaust systems are.