Tires are the center of handling since they are in contact with the pavement.

87s are wider than stock 3.0 rims. Stock tires on 87s will have a broader contact patch increasing cornering capability. The stock tires on 87s have much shorter sidewalls than the typical tires on 18s. That also helps handling. With all other things being equal 20 inch rims will result in a softer ride than 18s.

Increasing unsprung weight has negative impact on handling, however, since an X5 is so heavy it takes a considerable weight increase to change the percentage of unsprung weight enough to matter or be noticeable. As an example if the unsprung weight changes by 100 pounds the percentage only changes about 1%.

Aftermarket suspension components almost always result in a stiffer ride since most are for more spirited driving where the key is keeping the entire tire patch in contact with the road more of the time.

I would certainly expect changing nothing more than the rims from 18s to to 20" 87s with tires such as a Continental extreme contacts would result in a quite noticeable improvement in handling and result in a very similar ride. Turn in would be better, stability at speed would be better. The vehicle will be more fun to drive and one will feel more confident and likely more relaxed. Should put a big smile on ones face and justify the expense.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas