You're not using garbage brand gas are you? Here in Jacksonville, FL and surrounding areas, Gate stations are notorious for their poor quality gas. If you're not already using high quality gas, try switching to top tier (Chevron, Texaco, Shell et al.) for a while and see.
Also, why are you using 89 octane? This engine is made for 87 and it doesn't help to run 89 without a tune. If you have knock with 87, get a dealer tech to ride along and hear it so you don't waste your time and come out with them saying "we couldn't replicate the problem".
He can run 89 if he wants. The ti-vct will adjust for higher octanes. Performance will be increased with higher octanes stock. This has been proven on bama's tuning/intake video. This engine was allowed to run for 87 octane but IMHO it's too low of an octane. I had slight pinging on two tanks of fuel and it was top tier fuel. The manual informs drivers not to be alarmed if they're experience slight engine knock. I now run 93 with a tune and have no problems...