Sorry Gavin, I should have been clearer - I mean pads that pack out the space between one (or two at most) coil turns to increase the spring rate (by eliminating those lengths from compressing). There may be another term for them.
My thinking is that you could pack a turn or 2 and then adjust the height back down to get a higher spring rate for motorsport use and then remove them for everyday running for a softer ride. Pulling struts out all the time to change springs is too hard :P
Thanks - very tempted now.