NJ,
Did you happen to pull the plugs and look at the tops of the pistons? The only cars I have seen that cannot take more than 10* up top had fuel delivery issues, this includes my car, which had a bad injector on number 3. Piston tops told the whole story in my case, 3 were soot black, one was lightly tarnished.

Then again, I've only done one 91 octane tune for a guy that was moving out west.