What prop is on the boat currently? Does the boat hit the rev limiter? Does it come on plane well? If the boat isn't hitting max RPM, going to a bigger prop may not improve performance. You may want to look into some of the Stainless 4 blades out there. They hook up well coming on step without blowing out, and do well on top end. If you are running a 3 blade now, you would run a slightly smaller 4 blade ( if your hitting redline) due to thier better efficiency. I changed from a 3 blade to a 4 on my direct drive ski boat. The boat pulls much harder getting on step, and didn't loose any top end. Just better overall. Go to the Mercury Marine website. They have a prop selector program that might help you out also.
Darrell.