Well I already have a 4 inch lift in the rear of my XK so I'm half way there. Getting the front components some day could be a possibility. So like you, if it ever were to happen I would probably run a modified Superlift and net about 5 inches of lift. Again I don't know if I'll ever get the front components but if I did it would be nice not to have to go out and buy all new struts and coils once again, I can just do it right the first time.
I do like my HD springs but think if I were to pair them with the Bilsteins at the lowest lift height my cv's and tie rods will be at a greater angle than my liking. I figured since you can gain 3/4" of lift from the Bilsteins at the lowest setting that the MD springs would work nicely. Especially since I could always cut them and bring the spring rate up closer to the spring rate of the HD's and just adjust the struts if more lift is needed once they are cut. I would definitely not want to do any cutting of my current HD springs since the spring rate would go up even more.
I don't think I'd ever spend the money for coilovers on this XK, maybe when I get a Hemi. If I'm spending any great deal of money on the 4.7 it's getting RCV CV Axles.
Why is it that when you cut the springs the spring rate goes up. That's another thing I really never understood