I don't know how much another 5 percent would have made of a differance but they suposedly run 15 percent gasoline minimum to give it some lube and keep from hurting rings...

Also remember that lead clogs up AF meters in the exhaust and other types just arn't as acurate.

As for if it was detonation I would think it could have happened during the gasoline run and not showed a problem till you switched to the E-85.

The 105 octane rateing on the e-85 is kinda a misleading way of judging it's usefullness since it cools so much when it goes in the engine it kinda artificially increases it's octane even more.