Relatively easy to answer question. Take your dyno sheet and overlay it onto the impeller charts found at http://www.liquidaddiction.net/impeller.html and grab the biggest impeller you can that doesn't intersect too far down on your RPM from your dyno sheet. Basically if you like leaving harder than having top speed numbers move to a larger impeller than what you have. Not knowing what you specifically want out of the boat makes it harder to answer the question. Since I think LV even had an 18' Biesemeyer at one point in time, he might remember what he was running and how it worked.
Good luck,
Chris