Don't lower it. You will destroy the ride quality as the X5 will be permanently riding on the bump stops. I know it is tempting to reduce the fender/wheel gap but you will regret it.
The 20's fit fairly flush as is. i would leave the front and maybe do 12mm in the rear. Make sure you get the longer bolts in the X5 diameter if you do (they are not the same as used on other BMW's and are kind of tricky to find).

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2013 Cayenne GTS Basalt Black/Alcantara (Wife's Daily Driver)
2013 Porsche 911 cab Aqua over pebble and agate (My daily driver)
2010 Porsche GT3RS Black Grey/Black Alcantara (Track car)
2012 Porsche CaymanR Peridot Green with carbon sport buckets (Wife's Track car)