Something else to think about D, is just because you're having to pull timing doesn't necessarily mean you're losing power...the opposite is true.

If you're running too much timing and the knock sensors are pulling timing, then you are losing power. You want the car to run to where it makes as much power as it can without any knock. Most cars I've had made more power with less timing. Every car is different.

Although I don't have a custom tune I fill up with QT gas almost 90% of the time. I get great gas mileage, usually averaging 22-23mpg on normal driving highway/city. I too stay away from Shell, one because its so damn expensive, and two because of the things Tim and Josh are talking about.