I sell these things and let me tell you I have people call me asking the same question. I tried it in mine and there was a serious difference/drop in performance and mileage as well. I refer to the owners manual in the Audi book and it states that there will be a drop in performance when dropping the points of the octane.

I have learned to curtail the use of the A/C, drop the windows at lower speeds and raise them as you go faster. Or if you Need the A/C use it with the recirculate button and cycle. Meaning, when the interior temp is comfortable turn the A/C button off and the recalculate button on. After a while the interior temp will drop and you simply repeat this process.
It helps a lot let me tell you. I've tested it several times. But the best way to save gas is to roll the windows down a little and cruise. Especially if you're the only one in the car.