I have a suggestion. Since a few of you have bought ScanGauges, it might be worth using them to characterize engine behavior on various fuel grades. One of the instantaneous readouts on the SG is spark advance, which varies depending on what the knock sensors tell the ECU. My recommendation would be to use that readout while running through a tank of regular fuel and get an idea of what the spark advance looks like under a variety of conditions. (It does jump around depending on what the car is doing.) Then run through a tank or two of higher octane fuel and see what happens. Increased spark advance on high octane fuel would be an indication that the car is having trouble on regular.

I did this experiment a couple of weeks ago and came to the conclusion that higher octane ratings don't help me. YMMV.