A couple things to consider.

Using rolling diameters different than stock will affect the speedometer - reading higher or lower than actual.

Driveline damage, as it relates to using different rolling diameters front to back, are dependent on your model X5.

2003 and below (non-xdrive) have a locked front/back torque split. If your X does not have xdrive then match your front & rear tires. If I remember correctly they can only be within 3-4 revs/mile tolerance difference.

If you use a site like this Tire Size Calculator, you can compare tire specs. There are differences between mfgrs even with the same sized tires. So, while the dimensions are nominal, and not actual, they are fine for our purposes.
Tire Size Calculator - tire & wheel plus sizing?

According to that site your current setup has about 21 revs/mile difference.

2004 models and up have x-drive. These allow for more tolerance as the torque split is not locked. However, I can't remember right now what is the tolerance.

What year/model is your X?