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I don't see why lower octane would cause it to downshift unless it's severly affected by something else.
On the other hand, it kind of defeats the purpose of getting better fuel mileage if you have to pay for premium fuel.
Octane shouldn't be much of a difference at lower engine loads. At lower loads engine timing and power output have more to do with just finding the right ballance. At idle you should be able to advance the timing as much as you want and not cause any detonation.
Where octane is important is at higher engine loads, like when you have the pedal to the metal. But shifting from 4th to 3rd at part load because of using the recommended octane instead of high octane fuel for forced induction engines sounds fishy.
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