[Adger Smith]

John,
I deal with your situation with quie a few engine customers. I can say that with out a doubt the low octane race fuel will be more consistant. The ethanol mix is never the same. It is mixed by vol. When tuned (jet & timing) correctly for the race fuel the engine will like it and will usually make your car more consistant. I can also tell you that after many years of refreshing my customers engines the engines run with race gas will show less wear, esp. in the cylinder and valve/guide areas.You are absolutely right about the higher octane not showing you anything. Your cylider pressure is too low for octane to be a real issue.
One recent example of the difference in pump gas and race gas was a 540 I built and had on my dyno. It was built for a Velocity 42 ft I/O boat. It had to idle at a low rpm to be docked and run in no wake areas. It was a 9.5-1, hyd roller, Brodix head with an 850 carb. It only had 155 lbs cranking pressure. I broke it in on VP C-12. (nice to have a little lead on new parts) I also did a Jet and timing loop while on the C-12. Then I dropped in some Shell 93. It dropped 25 hp. After a jet & timing change I got it to be only 13 HP less than the C-12.
It ended up making 650 lbs ft of torque and 625 HP. The torque curve carried 600 lbs ft from 2,800 to 5,600.
I don't think that was too bad for pump gas in a less than 6,000 RPM engine that will push the big boat over 80 mph and will have good driving manners. This was the third engine the customer had built for this boat. The others expired because their builders didn't take into account the use of pump gas and the way the boat was used. (stuck on selling big HP numbers)
Your combination can run the low octane fuel. good luck with it