My biggest issues with going with the staggered - which is how this set is available - are 1.) inability to rotate front to rear; and 2.) additional unsprung weight. (And I'm trying not to make tire $$ a deciding factor here)
Increase in unsprung weight should be your number one concern. The increase in rim size means a heavier wheel unless you're using very pricey forged wheels. I dont' know what the brake system changes are in the X5 model line, but especially with SUV's SAV's, they will suffer in the braking department when using a heavy wheel if you've made no other changes. Will you or your wife notice? Odds are not most of the time, but in emergency conditions I think you've reduced some margin.