For your application the factory aluminum impeller is more than sufficient. If you start building horsepower north of 600-700, then you should seriously consider a bronze or stainless impeller.
I would recommend starting with the Place Diverter and see how your hull reacts to the adjustability. If you find yourself running the diverter down to keep the boat from porpoising (bouncing the bow) when cruising, you will find that adding the wedge will reduce the amount of down motion in the nozzle, and make it more annoying when you aren't playing with the roost.
As for the question about "impeller size"... for best results the impeller needs to be matched to the engine and unless you have a serious "big budget" build going on, you'd be better off just running what you have and have fun with it.
