I believe it's characteristic of the X as it is heavier. It got "worse" when I went to the wider 20s from 19s, probably also due to weight. If you inflate them more it may help a bit.

Some time in the past I noticed even my other vehicle was more responsive after I got new tires - I was way overdue - balding tires = more contact patch = heavier to turn... same concept with wider tires (as if weight didn't hurt already).

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr