i am going to be running the greddy evo exhaust 80mm b/c my GS is turboed now...i want to get rid of my cat con...and replace it with a test pipe..how hard is this adaptation for the piping?? so ill have 2.25" running into the test pipe (what size test pipe should i use?) and then the test pipe running to the 80 mm exhaust...anyone know os a way for adapting these pipes?

One end of the test pipe is going to have to be 2.25" and the other end is going to have to be 80mm. I think if the numbers are close it will work. Muffler shops usually have adaptors that go from one size to another. You could get 2 flanges. One that is 2.25" and the other is 80mm. Then get piping that fits both and get a exhuast coupling that can join the 2 together.