2mm will make the different already. More air to go in to the air chamber and combusion room. That's why some bore it until 64 or 67mm... After market TB like Jun is 70mm at inner diameter. But bigger doesn't mean better, big enough for ur engine OK lar. Don't tell me a stock b16a is going to put a 67mm or 70mm TB.
Anyone know this : The exhaust housing from RB20 can be changed to match the RB25 unit? If so, theoritically, the RB20 exhaust + RB25 inlet housing match would spool faster? Thank you