Hey Dman, I could use a little more info to really help narrow down the problem. It definately sounds like something is off though.
Let me see if I can help with what is provided though. I will guess the engine is a 1999 saltwater yamaha 115 2 stroke. If so the gear ratio is 2:1, and the WOT rpm range is 4500 to 5500. So you are bumping up against the max wot rating, which is good. (I assume you are not hitting any rev limiters, not sure if they were even on those engines)
The biggest question is what prop are you currently running? (pitch and diameter should be marked somewhere on the prop) With a gear ratio of 2:1 and running at 5500 RPM, even a pretty low pitched prop (lets guess 15" pitch) should be getting you in the neighborhood of 35 mph if you are seeing 10% prop slip. My guess is you are seeing massive amounts of prop slip. First and foremost, you are likely going to want to switch to a 4 blade SS prop. Most single engine cats will perform much better with a 4 blade prop versus a 3 blade. I would also look for a 4 blade that has a decent amount of cupping.
If you can get a good estimate on your boat weight, current prop specs, and some good numbers on current performance; give the guys at propgods.com a call and get their advice. My guess is you are going to want a 4 blade with about 14" or 15" in pitch, but that will greatly depend on comparing to what you are currently running. Hope this helps, good luck.