So any mpg benefit for using premium? I've noticed that the mpg is about the same if not worse with premium.
It's actually common to see this happen. The lower octane burns differently which changes the timing in the motor. It may increase your MPG in the short scope but in the long scope is not worth it (to me). Honda designed it with Premium in mind, the manual says to use it.. I would trust the Honda engineers personally, as they have been tested and proven to squeeze thousands of miles out of a motor.
Like I said your ECU will compensate for the lower octane fuel but it's not what your ECU is anticipating to add to your motor. I know it's hard because you wont see any direct benefit but the higher octane is used to prevent detonation, I think you'd probably still squeeze plenty of miles out of your ILX but it's well worth the additional cost to use what Honda has set for the car.
I mean let us consider this.. It's on the fuel tank, the owners manual, and the door.
Hope this helps.