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.
2.0L AMS Performance's 1035hp 4G63 won the Castrol Syntec 2008 Top Shop Challenge by spanking Cosworth's 3.8L VQ35DE and only 'lose' to a Chevy Big Block V-8 7.0L (with three times less hp/L figure)...