Avia Prop
Russell's 6A was the first one I flew when I started shopping for a completed RV. I can attest to the ease of climbing high using the Avia prop on an O320. We were loaded pretty good and had no problem climbing to 12,500msl. It takes a little longer for the Avia to kick in on takeoff than a Hartzell but it performs much better than a fixed pitch on takeoff.
One thing that alarmed me was the suggested TBO on the Avia. The last time I read the specs on their website it was much lower than a constant speed, but, if you have a solid crank engine, it's the best you can get to improve the performance.