IO-360 RV-3
Mine had a 360 in it for the first half of its life. She was wicked fast, raced at Sun-n-Fun and Reno at speeds over 225mph. It was flown aerobatic with the 360 and CS prop, but the weight would not allow for much fuel.
It now has an IO-320 160 hp and a fixed pitch prop which has lightened her up some, empty weight 845 lbs.
If you are building, and not planning to race, I totally agree that a build with attention to weight...like count every ounce and forget all the fancy avionics and ****, and a well built IO-320 and a Whirlwind composite CS prop would be unbeatable in performance and flying pleasure. If I were building or buying...that is the direction I would go.
Lighter is better without a doubt, empty weight under 800# with 160 HP is unbelievable.
Best
Dave