if your used to using low grade fuel in a car thats suppose to have premium than you will probably feel a bit more responsivness. its not very noticeable, but try going to low grade for a few tanks and then youll see what im talking about. some part of it is a mental thing, the other is that the car is performing to the best of it ability now.
always use the recommended gas that BMW prints in their manuals. its a wise investment as well as a good piece of mind.