I've heard that if your ski is stock, you might get 1 mph on the top end from the 17/29. But if you're running that impeller with the Macsboost or Jim's Alpha Kit, then you should get another 3-4 mph. I haven't heard of anyone running an 18/30 on a Honda.
I just see their is a offering for a 18/30 prop for a Honda, and wondered if it would be a big benifit with having the macsboost mod. if i'm going to spend the money would like to get the best prop for it. Its $300.00 either way. So want to spend it on the best performance I can get.
If you go with the Macsboost kit, it comes with the 17/29 impeller. (I'm assuming you only have the module itself.) You're best bet is to get a hold of Shawn directly, tell him what you want, & he'll tell you what the best option is.