What make, model and size is on there now? Sounds like pretty good performance already.
I currently run a Yamaha Saltwater Series 15.25" X 15 on my 150 HPDI and I picked up a Yamaha Performance Series 14.25" X 18 to try out this season as well.
gf I had a 15.25x15 alum. on it and got very good service from it but did have it fixed twice do to lobster pots. This last hit one of the blades tore and I just think I may get bet resultes with the sst in this case.