if you fit a 1.8 carby downpipe a gti system will fit straight on, but probably wont see too much gain as you still have the restriction at the manifold. you can however fit the gti manifold and downpipe, though space is limited back there. if you split the manifold and downpipe it should go in ok though. this will see a noticeable benefit, I put a complete gti system on my driver and it mas better, little quicker and more happy to rev

only trouble is actually getting the manifold off, one of my studs rusted away to a small lump and in the end i had to have the head off to drill it out. ended up a good thing because most of the coolant holes in the head were clogged up wi crud