I am running the stock exhaust pipes with a straight pipe instead of the muffler. If I installed a 4in sytem from the turbo back would I notice a big difference in power? Would I be wasting my money on 1/2inch bigger pipe? I have a hypertech programmer and brute force cold air induction on my 2002 duramax.

You most likely notice any power improvement by going to a 4". What you should notice is cooler EGT's.