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Long-time track and autocross driver here - so I've done a decent about of tire-related research over the years. You won't risk hurting xDrive unless you put on mis-matched tires, like different brands front and rear (unless they came staggered in size from the factory - but even then, I'd never put on a different brand or type of tire on one axle).
xDrive wants to see similar circumference on all wheels. I rotate my wheels every 10K to even out the wear (using a simple front-to-rear swap, not the more complicated front-crossover method that requires all four wheels off the ground).
Often, OE tires are some of the worst available when you look at Tire Rack ratings. My X5 came with Bridgestone Alenza Sport A/S tires, which are rated #32 out of 54 for my tire's type and size. I definitely won't be getting these tires again, although admittedly I personally have no complaints with them so far. The priority of the X5 is comfort, so the tire requirements aren't very stringent.
Those Michelin PS4S tires are awesome summer tires. I've had those on several of my "fun" cars, and may go back to them in the future. Right now I'm running Cup2's on my fun car. Great tires when they get heat in them (like at the track), but for the street, the PS4S is a much better all-around tire. I just wouldn't recommend the PS4S tire (or ANY summer tire) once the temps drop below 40F. In light snow or freezing rain, they are comically ineffective as tires! When I was much younger and naive, I thought I'd be OK on summer tires in the winter if I just drove extra slow. All it took was a couple flurries to prove that theory wrong!
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