[GP20]

The H&R springs I had were stiffer than stock but not harsh at all. They improved the handling very slightly. The thing I didn't like about the H&R springs was that their claimed 1" drop was more like 0.6"". The front wheel gaps still had a little more than 2 finger widths and visually they just didn't make enough difference.

And what do you mean that coilovers would be way too aggressive and way too low? If you want the best ride comfort, coilovers are the way to go (TCK or KW V3; don't bother with H&R coilovers which are crap). Lowering spring+OEM struts/shocks would never handle as well as a good set of coilovers with adjustible dampenings on a car like the Z4M. And you don't need to set the ride height low either. You can set the ride height the same as stock if you want, or basically play with the height settings until you're happy with them.