Too bad the SL uses the stock springs/struts... so i'd have the same issue even if I did get the SL (yet another reason I wish they would give us spring/strut upgrade options like they do for other vehicles that they have lifts for!)
I was thinking about it today and think that the OME MD springs may actually be the best solution for me. Since I have a 5.7L XK (heavy), the MD springs would probably just compensate for all my added weight and therefore also work with either the RC spacers or SL (in the future). Kind of like if SL offered a spring upgrade.
This may be a good reason to replace/upgrade my 4+ year old stock struts... get a set of the Monroe Reflex (quality monotubes) and a set of the OME MD springs. Actually would get some fresh mounts/isolators and just assemble everything outside of the vehicle so it'd be a quick swap in. Then with that, I should be good to go next year when I plan on installing the SL and none of the parts will go to waste.
(Edit: just noticed that the Reflex shocks are monotubes... it looks like the struts are low pressure twin-tube, so i'd probably go with Bilstein HDs)