Just do some research into which type of NOS kit is best for the Golf. Dry or wet??With the GTi-6's, wet kits are the way forward. I think your best off speaking to some tuners.Which turbo engine have you got?Have you also sorted your suspension( inc. rear ARB)/brakes out? Having a sorted susp. setup can be like having an extra 50 bhp (improves your point-to-point pace). Do you do track days? You could also get some instruction from some qualified curcuit instructers who will teach you how to extract the maximum out of what you've already got!!
I would just go bigger turbo rather than nos tbh