First off, you must remember that you started off with 120kw, you might be at 130-135kw now and golfs are know to be under estimates on power, so he might be at 160 kw stock. (All crank figures). He also has at least 80-100 nm more than you have so you realy need to catch them in a wrong gear.
Where are you getting a detroit tuned bpv? Mike on here has one for you for chips, might be worth looking at.
If you want to try I have a full oem exhaust that you can do a one ball mod on lying in my garage. you can get it for free if you pickup. Then you can remove the other cat and have them mod the exhaust as you please.
To stick with gti btw, I would add a filter and then do programming. other thing is to get launching perfect.