exhaust to my GU

G'day

i want to put an exhaust on my patrol and was wondering what size gives best results 2.5" or 3" .? my patrol is completely standard at present, i have future plans to put a motsons enhancer in , i have also heard about blocking the EGR off , can anyone give me the up's and downs about doing this and where its located and how to do it successfuly . Does anyone know what boost the 1998 2.8ltr Tdi runs? and if you can what's the highest boost you can run ?

I'd like to thank everyone who can provide me infomation about the above in advance.

cheers
diesel dave