So i recently bought a b5 2001 1.8t 5spd. I did a custom ecu upgrade, boost gauge, oil pressure. nothing much yet. I just enjoy doing fun bolt ons. I am thinking of getting a test pipe, i heard it is the most restricting part of the exhaust, do you think I will notice any gains? Also, I removed both mid muffler and the rear muffler and created a Y pipe so it came out as a split like stock. I am very happy with the sound, and would like to do the test pipe, but will there be enough back pressure for the stock ko3 turbo? it still has the stock sized pipe and the resonator

 

I believe normal gains are 10-15hp with your modifications.

 

You'll be fine on a K03 with a test pipe. The turbine itself gives pretty much all the backpressure the engine can use, and de-restricting the exhaust after the turbo is generally the best way to go for power/response improvements.