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There is an adage that correlation does not imply causation.
I don't agree with your statement that the rolling circumference in non OEM tires is not most friendly to BMW's xDrive. The issue with all wheel drive vehicles is that one wants all of the tires to have the same circumference. Sounds like the summer tires met that condition.
The biggest change with a set tires with a slightly different rolling circumference will be the speedometer and odometer reading.
That said tires of different types with the same base sizing can have a difference in rolling circumference. It might not be much difference but there can a difference. I did a random selection of tires from four different mfg. that are 225/45R17 the revs. per mile were listed at: 835, 832, 833, 831.
That said, suspensions are set for run flat tires so certainly running go flats will have some affect such a softer ride.
At the end of the day changing tires improved the handling but I would not base it solely because of OEM * tires. Probably a combination of small changes, new tires, run flat (stiffer), winter (softer rubber), etc.
The question is come spring what are you going to run?
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