I just switched back to a 15x13 on a mojo with a 400 from a 14.5x12 (acme 2079). I couldn't be happier. Lowered my surf rpms and allowed me to surf faster. I was running 10.8mph at 3900rpm now I go 11.1mph @ 3500rpm with a harder/longer port wave. Note: these are skim speeds, surf style speeds are faster for better airs.

The engine seems happiest below 3500rpm but that is just my opinion.