I put a 4 blade Mercruiser Vensura stainless prop on my 18' I/O and was very happy with it. There are plastic caps just behind the blades that you can drill or replace to vent a bit of exhaust at slower speeds. That allows you more slip on the prop at slow speeds which helps acceleration. I tried the High-5 also but it was not better for my boat.
The improvements over a 3 blade stainless were better acceleration, better ability to hold speed, and less likelyhood of cavitation on corners. I'm sure it would be worth it, especially if you are using a 3 blade aluminum now.
Make sure you figure out what pitch you want your new prop to be.