I have the Cosmic Pro Carbon SL UST wheels. I’ve never ridden the non-SL wheels, so I can’t comment on the comparison. I do like the SLs, though. They feel solid and the braking is reasonably good; still not like good pads on aluminum, but no problems. Every once in awhile I do rides with long, steep descents and while I think about the potential for overheating, I’ve never had any indication of it happening. For reference, I’m about 68kg right now.
They are noticeably heavier than my carbon tubulars, but I find these wheels good enough I pretty much don’t use the tubulars any more, even in races.
It’s worth noting the freehub is quite “stiff” when new; with your bike in a stand, when you rotate your pedals backwards, the wheel will spin backwards. After awhile that stops happening, though I guess if it really bothers you there is a bearing shield or something you could remove.
Overall I’ve been very happy with the wheels. $800 Euros seems like an awful lot for 200 grams, but given that they’re not super light wheels it is nice to get the slightly lighter model.