Do you really *need* a change in suspension set up? And if so, do you *have* to have it done now?
The answer in both cases is no.
I've driven my car (usually set up for Yokos) on slicks and there was still a suitable performance improvement over the road tyres. OK, suspension tweaks may well have made it even quicker and, possibly more progressive (though my feeling that the slicks weren't could just be how slicks are), but there was still an improvement and the car was still safe to drive (though some spectating at the time might have felt differently - then again, they do when I'm on Yokos).
You could opt for some Yokos and leave the suspension. You could even have a bash at fiddling with the suspension yourself (I believe Peter Carmichael [i seem to be refering to PC often these days, but he does write a lot about tech stuff] has posted in the past on his DIY flat flooring technique which is easy enough to follow), tough the rears do ideally require new ears.
21Rs are way better than CR500s in the wet and in my opinion ("feel" is always subjective remember). CR500s are good, but Yokos cut through standing water better than anything I've tried. It's one of the main reasons I use them as I'm not partial to cacking my pants (though in winter there are warmth advanatges) and aquaplaning at speed is one of the things in life guaranteed to give me a better bowel movement than a grannie on sennakot.
And with the difference in cost of these tyres, your flat floor session will have been paid for with 6 tyres anyway.
The ideal combo has to be a road tyre (21R/CR500, whichever suits) and slicks for track use. Then buy a wheel rack from Peter Carr to allow you to transport the wheels on the 7. Note, however, that Richard Ince doesn't believe there's a quantum leap between ACBs and slicks so you coul djust as easily go ACBs (both are available "scrubbed in" for not much per corner...
Of course this is far from the cheapest solution, but it's a good one.
Incidentally, it's possible to rip both CR500s and 21Rs to pieces on track in the dry. I got through a set of 21Rs in about 1k road miles and three track days on Le Sept. The rears, however, were a bit too narrow for the car and they were getting pushed at times.
Biggest reason for Caterham flogging CR500s is their tyre deal with Avon...IMO of course.