Black specs are not detonation. Look at the inside of the shell and see how much carbon is built up on it. As a general rule, those black specs are bits of carbon coming off the shell and landing on the porcelain.

At your compression ratio you shouldn’t need any octane booster. There ain’t any power in octane and you only need enough octane to stay out of detonation. 91 pump gas should be more than enough.

EDIT: lewtot184 pretty much nailed it. Just wanted to add that. You always have to tune to the leanest cylinder but anything you can do to clean up what distribution issues you may have will always make more power and the engine will “drive” better.