I think you are missing the point of large rim diameter. It's how you run a lower ratio tire on the same tire diameter so you can get higher cornering performance and lower unsprung wheel / tire weight and all the helpfulness that comes with.
Consider this: BMW chose from all possibilities to use 285/35/21 on the front of their highest performance X5M. Do you think you will achieve a better solution with "seat of pants" engineering?
The combination you chose is about 6% larger diameter and I'm pretty sure you will find other problems than a large speedo error.
It's not an accident that all the BMW approved sizes are within 1% rolling circumference.
https://www.carbibles.com/calculators/
Use the tire size comparison calculator to find a size you'd be happy with that is within say 2% of the original circumference.
Example: 255/55/18 and 255/50/19 are identical diameter.
255/35/22 is the magic combination that nets you the same diameter.
265/35/22 is a decent match
255/35/22 is an exact match
295/30/22 is an exact match
305/30/22 is likely an ok match to the 265/35/22
If I was going to set up an X5 on 22" rims I would run:
255/35/22 front 295/30/22 back. They are within 0.2% the same size and very close to the design size for the car.
255/40/22 & 295/35/22 is about 4% bigger than stock. I would not recommend larger than stock by more than 1% though, your affect all dynamics of handling, for what purpose?
If you want a softer ride with trade off of lower cornering performance, the alternative set is:
235/40/22 & 265/35/22.
There just is no good reason to upsize the wheel and tire that I'm aware of.
If you are aiming for a lowered look and fill the wheel well for a "low rider" look with the trade off of performance, that's a reason but the trade off will be rubbing tires on the wheel wells and worse handling.
275/315 fits in the wheel wells of the X5.
Actually 285/35/21 & 325/30/21 were used on the newer M model. That's actually smaller rolling circumference than the base sport tire.