I agree with 56OctyVRS. A couple of years ago I actually installed a 3 inch Milltek DP on my k03 gti mating to the oem catback.
The theory was get more top end knowing I could afford to lose a little bottom end. I was stage 2 at the time. I did get a bit more top end but lost a lot of low to mid so I totally believe the 3 inch is to big for the k03. Needless to say it came straight off.