I agree with most of the guys that the air intake and exhaust header should make a small but noticeable difference. With no mechanical mods I just started a new diet of 93 octane non-ethanol and noticed a small but satisfying increase in all performance parameters. Of course, you have to have a reliable source for such stuff. If you live near a large lake frequented by power boaters, you should get lucky and find a filing station that sells good quality juice. Your engine controls will make enough of a change in the tune, and your boxer will love you for it.
