I thought about strut braces a lot when I had a Zed 3, BMW had one listed for my car at a cost of £330 I think. If BMW make one to fit on the Zed 3 I don't think the insurance companies can say much about it as its regarded as an after market upgrade by BMW. That is of course if you buy a BMW Brace. In the end the only time it might have made a difference, I decided was on the track at Silverstone and I didn't miss it then although we were traveling at speeds in excess of 100MPH So I never did get one. I've not looked to see if you can buy a Strut brace from BMW for the Zed 4 E85 as i never felt mine needed any enhancement. The suspension was very stiff on the E85 and the car handled very well on M sport Suspension. The E89 is a different kettle of fish, the springs are very compliant and the chassis very stiff. i'm sure if i corner hard I can lift a front wheel. But that requires a different solution.
Mike
Mike