First the best way to improve performance is with the correct prop. You can try all kinds of different props with different types of blades styles and cupping and diameter and weight in the boat and water and wind condition and you will always get a variety of results but the only way to know is by trial and error.
Also in the marine industry weight is everything you will loose significant top speed as you add weight to the boat (fuel, passengers and cargo). Many factors can reduce your RPM in many ways: upgrading to better materiel, increasing the diameter and/or pitch, upgrading to a 4 blade and cupping, just to name a few
The difference between 3 blades over 4 blades is that the 3 blade (smaller blade ratio) usually goes faster and the 4 blade (higher Blade ratio) is better hole shot and better stern lift as well as better handling and fuel economy because it will keep you on plane at a lower speed. If we are talking performance; stainless steel propellers are the best followed by composite propellers and then aluminum.
If you are worried about hitting i suggest a composite propeller by Piranha. Piranha composite propellers are superior to any aluminum and are designed to save your lower unit. Piranha propellers were developed with three things in mind: durability, performance and affordability. A key feature is that the blades are replaceable. You can change the diameter and the pitch, save on the costs involved with propeller repairs, not to mention the potential damage to your gear case. With Piranha you can have some spare blades in the boat for a quick and affordable repair.