If you don't mind compromising some of the ride quality, you could cut the springs a touch or heat them. It is a very old-school trick, but it can seriously alter the spring performance. Plus, given how low the frame rails are on the E9 already, I would be wary of going too low. You go over a speed bump at anything over 5 mph and *bam* you just bent your frame rails, or at least started a new rust location.
My E34 is like that now, thanks to my father trying this trick. I have to be aware of how much ground clearance I have at all times, lest I crunch the frame rails. Speed bumps have become "stop bumps". Driving the E9, it is nice to actually have some ground clearance.