All US x3ms had HK standard so not really a comparison. When I get an x3 loaner that stock stereo is noticeably worse. As far as HK performance you need to adjust the equalizer in the settings and then it's reasonably ok, not great but not bad either.
As far as 20s vs 21s, 20s will have slightly better ride comfort, less traction due to slightly more narrow tires and slightly more body roll. 21s will be the inverse. Slightly better traction and less body roll with worse ride comfort.
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