A US spec S500 has leveling suspension effected by the struts. Softening the springs such as with air bags will make the hydraulic system work harder to maintain the preset level. I don't know if the struts can hold the body up without springs. If they can, then the car won't ride lower.
Seems to me that the alternatives are to adjust the leveling valve for a lower rear ride and lower the front springs mechanically, or get the more conventional rear suspension of an S420 under the S500 and go with airbags.
Under what conditions do you 'need' to raise or lower it?