Skat-Trak's website recommends a 15/20 Swirl. Unfortunately, you won't gain any top speed with a Swirl impeller. It's made for bottom end and you will lose a little top end. The Sportster, having a single 720, is underpowered. If you put an impeller in that will gain top speed, you will hate the amount of time it takes to come up on plane.
Chester