Well, I don't know of any places that offer the 63mm TB (SHOShop no longer sells it). I did some research a little while back on this subject and came to the conclusion: While optimizing the TB nets some gains, the TB is not a major restriction in the intake path until you're pumping in a lot more air (3L, forced induction). So optimize the 60mm TB and be done with it.
HTH
