I have a custom painted Whirlwind RV-10 prop on my RV-10 and I love it. It's my second Whirlwind prop, the first was a STOL 200CS installed on my Glastar.
Over 10 months I have about 160 hours on mine. The performance numbers are a little better than Van's book numbers but I can't say it's directly attributed to the Whirlwind prop. However, I am confident that the prop is not hampering performance. There are so many factors that can affect cruise performance right down to how people manage the engine and I haven't run any other prop on the plane for a direct comparison. This is not a race plane so +/- a knot or two is not going to make or break you on even a long cross country flight. My longest flying day was just over 1500 miles and 200mph or 207mph would hardly be noticeable in real world flying. My main reason for going with a whirlwind prop is the light weight. It allowed me to mount the battery on the firewall and eliminate the long long wire run to the tail while keeping the weight and balance where it should be. It, along with the EarthX battery, saved about 40 to 50 pounds.
There was a report on here by someone that they experienced some weird buffeting in the fuselage when they switched to the Whirlwind RV-10 prop for some side by side testing. I've talked to several builders that are flying behind a Whirlwind RV-10 and nobody has reported anything similar. A little while ago I had to remove the prop to replace the ring gear on the flywheel and I didn't mark the prop. The first flight afterwards I experienced something that could be described as a weak buffeting so I removed and re-clocked the prop and it was back to normal.
I have a Smart Avionics PB3 dynamic prop balancer and was able to get the balance down to .03 IPS which is about as smooth as it gets for a piston engine. With the IO-540 and the RV-10 prop it is an incredible smooth combination.
I'm bringing the airplane in for paint next month and the airframe will finally match the gorgeous paint on the prop. I hope it turns out as nice as the prop did. You've probably heard it before but these composite props are works of art and the RV-10 model is no different. One of the other RV guys on the field calls it propeller porn.
If I was to build another RV-10 tomorrow I would hang another Whirlwind on it. The factory recommended Hartzell is also an excellent choice but for me this prop had everything that I was looking for. I'm a very happy customer.