Gary, what pitch is your current prop? It sounds as though you are running a prop with too higher pitch which is not enabling your engine to achieve the Wide Open Throttle setting. This will cause the engine to labour and not run at its optimum.
You might want to drop a couple of inches in prop particularly if you are going for stainless. An alloy prop will flex so that in reality a 13" pitch prop maybe 12.5" under load. A Stainless prop is far stiffer and flexes less.
Ideally you want to aim for a prop that gives you the performance under the load conditions you usually use the boat. So if you are always out with 3 people and full fuel load, select a prop that suits those conditions.
Final thought, is that your engine is a 4 stroke which in my laymans knowledge of 2 vs 4 stroke engines probably doesn't rev as high as a 2 stroke? 5000rpm maybe WOT for your engine. Your manual will tell you.