This is the kind of question that can often get as many different responses as people you ask. However, despite your car having a knock sensor there probably detonation occuring at lower-levels that you are unaware of. I have heard many cars with knock sensors pinging. If I run anything less than 91 in my 540i, which is the highest octane you can get in California you will hear significanting pinging. Regardless, a 545i is a 60k thousand car and an X5 4.8 even more. Buying cheaper gas will save you a little bit of money over the long haul, but it probably isn't worth it when you consider the cost of your cars and the potential long-term wear.
I having a racing car and after my last trip to the track I had 5 gallons of left over 100 octane (5.95 a gallon). I decided to put it in my BMW when the tank was near empty. I feel a pretty massive difference...much fatter torque curve.
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