One of the choices that I have not heard discussed for the -9 is an O-235 - but with a turbo or supercharger. Should be very light, with a Airflow Performance FI it would be fairly straightforward to supercharge. Stick some 9:1 pistons in it, add electronic ignition, turbonormalize up to 15,000 feet or so, run lean of peak, work on making it as slick as possible and would be the most efficient RV in the sky while outrunning 0-360s at high altitudes.
If you live at high altitude, and fly there most of the time, you give up nothing. At an 8000 density altitude, typical for many western airports in the summer, The most we get out of an 0-320 is 120 hp anyway.
It all depends on the mission and with fuel prices where they are, this would be a cool, fast, lightweight, efficient cruiser!
As Paul pointed out, there is no perfect airplane, only the airplane most perfectly suited to mission. That is why we really need two of them!