"cold air intake" piping size

2x3" tubes will be fine. I run 2.6L Mitsu rally engine at 14-15" boost through a single 2.5" tube from the air cleaner into the turbo inlet at 275 HP. Never an issue with getting enough air.

Your engine will like the cooler air. Just watch the mixture. I am looking into the same thing for my son's 340.