I'm in the works of making a down-pipe for my t3/t04e turbo.... Most people say run a 3" dp, but I'm looking at the turbine which is only 2"(guessing), would it make a difference if I use a 2", 2.5", or 3" downpipe when the turbine exits at only 2"?

 

Yeah most turbos hot side is only 2", that is why most people and companies run it 2" and then after the flex section they open it up to 2.5" and 3". I think if you are going with a 2" or 2.25" it's just as big as stock and would choke it. I'm gonna go with 3" just to be safe and it seems to be the standred in sizes.