The rear will fit an E53 X5 fine. The front will need hubcentric rings.
Bore opening in wheels of X6: 74.1mm front 72.56 rear

Bore opening in wheels of X5 e70: 74.1 front 74.1 rear
Bore opening in wheels of X5 e53: 72.56 front and rear

I bought these exact wheels for my e70 X5. I am working with a local tire place/ machine shop to have them increase the bore size of the rears from 72.56 to 74.1 so I can hang them on my X5. If you look at the bore openings on the wheels, you will notice that the "ring/lip" at the mounting side (not wheel cap side) is ever so slightly thicker on the rear rim than the front one. This ring/lip can be "shaved down" by a total of near .75mm all the way around which will add 1.5mm to the diameter (.75x2) and should allow the rears to fit the e70. I have attached photos on another post. Search for "new wheels for 09" and youll see them there. Hope this helps!!