I lot of people misunderstand the grades of fuel some engines should be running on. Alot of engines run and make good power on just regular unleaded. Since you have a regular LS, non VTEC, you need to consider running on 89, or even on 87, and then recheck the timing. High octane is mainly used for cars that have either high compression or a high swirl combustion chamber, and the higher octane fuel has a slower burn rate. I would recommend dropping a few octane points. Your spark knock might go away.

BTW, what have you done to the internals? If you haven't done anything, go back to regular or mid grade. Just avoid premium until you increase the compression or boost it. It's basicly pointless in your car