I have read some threads that state differences in diameter can cause traction control and braking errors. Tread from new to bald would be about .68" so the X5 can handle near that for some period of time. I think you should be safe at about half that but I have not seen BMW transfer case tolerances. I wrestled with this and decided to go with a lowering kit on my 4.6. Not as easy to get there but does not mess with the computer, speedometer, mileage calc, or put greater load on the suspension. Plus is in spec for the transfer case. If you are only interested in going the tire route, I suggest you keep searching until you find a match. I don't know what component would be the problem but anything involved with the transfer case is going to be costly. If you are doing a lot of towing that is going to magnify the risk. If it helps my clearance between tire and fender lip is about 2 3/4' front and rear. Rims are 87s and tires are 275/40 on the front 315/35 on the rear, same brand, same diameter. I also have 7mm spacers front and rear that I added after I had the lowering kit in place for some time. Never any problems with tire rub. As you mentioned I replace front tires twice as much as rears so there is a period time where front to rear diameter is larger by about half the tread. Never had a problem with transfer case and have clocked a little over 100,000 mi.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas