#1- what 325I engine? M20?M50?
#2-when do you want full boost and how quick?
Just simply saying "BIG turbo" is pretty vague.
need more info!!!!!
PS. your Sig lists different shops that should have an idea about these things, do you own them or are you a part of them?
Searches help aswell as research.
Im glad to help but with the limited info and vague questions its quite hard