Common sense which is not so common.....
if you use common sense a full turbo back system allows free flow....which in turn makes the turbo spool up quicker, which then comes into boost a lot quicker hence extra power....you dont really get any/much power gains from a cat back system.....i love to find out all your findings mate....
I have a full turbo back system on my 33 and my mate has the same mods as me and running 2 psi more then me but he only has a cat back and is making less power then me.....on 9 psi he is making 175rwkw and on 7 psi I am making 180rwkw.....