Impellers...
I usually say, "ask the pros".....
There are many of us who could offer advice on which progressive pitch impeller to either get better bottom end or top end. I have two solas concords and really, to be honest, for the couple few hundred bucks I have in them, the tiny bit extra just doesn't justify the cost.
I used Impros to get mine. I felt wiht these guys doing this for a living, they'd know the best pitch for the hull and motor I use. You can also check out RIVA.