The runflats have a stiff sidewall for a reason. They have to be able to hold the car up with no air pressure. The disadvantage of a non-run flat is if you do get a flat, you have no spare.
You could put a set of coil overs in, and go softer on the spring rate. The spring is what holds the car up, and is what is directly responsible for the ride. Dampening only goes so far with the ride. The compression and rebound dampening are there to control the spring.