If your acheiving 5000 rpm WOT, which is the max rpm for your engine like youo say, then in my opinion it is proped out correctly, changing to a stainless will reduce the revs from your ali prop as there is less slip through the water. If you are happy with the performance of your setup, then i would go for a stainless prop of the same pitch and size of your ally, or possibly drop it down a pitch to a 16.
I would recommend
Steel Developments, they are very helpful and occasionally have some good offers.
Mark