Did this to my rear air system. Used tourtech springs with my HD Bill's n love it. It handles little road bumps and uneven surfaces very smoothly without any jarring response from the springs. Going of large pot holes, bumps and road imperfections I feel it as much as with air, but it's different in that you hear the rear more over the bumps so it gives a different impression. I didn't notice any lack of body stiffness around corners, as the air system is weight sensor monitored and not sonar road reading response. BTW, I have a HD Bill's on the front strut setup.
Depending on age and dollar amount needed to fix it you may like the coils.

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X5 3.0 AUTO -2001 - 238,000 miles