I had this very problem when I upgraded my turbo and tried many different things like reducing the silicon down to meet up with the factory silencer etc etc custom silicon but now I am just using a 4" flexi pipe from auto pro which does the job, you loose alot of what you gain if you don't delete the factory silencer as it is only 3" in diameter and severely restricts flow into the mouth of the turbo, I tested it with my car and it lost several pounds of boost above about 4000 rpm, was like hitting a brick wall.