Very interesting - just happen to find this:
http://www.bimmerforums.co.uk/forum/...-wear-t128920/
My wife also ohas an E70 X5 with 21" wheels and we were getting excessive wear and went through a set of four in the 1st 12 months. Because of the cost of the tyres I thought I'd look into the problem of excessive where and contact BMW directly as I have contacs in the Automotive industry working there myself.
I spoke with development engineers and designers and it turns out that to prevent excessive wear on all Msport cars the tyres should be ran at there maximum pressures, that means 5 adults and luggage setting as detailed on either you fuel filler flap or drivers door b-pillar.
This is a know issue within BMW and this is what they recommend because of the excessive camber angles and other geometry settings. It is by no means a total solution but coupled with regular checks which should be at least fortnightly (tyre pressures) if not weekly it does extend the life of the large diameter tyres.
Its a shame BMW don't make this public knowledge and it proves its definitely a case of who you know not what you know.
So, it seems like air pressure have something to do as well. Despite I increased my air pressure before the long trip, I did not increase it to the max of the tire.
Maybe this is something to try - I actually also find the car (any car) is driving better with high air pressure