I've had no issues with the supershelter myself, nor has my son Weasel. We've both used it down to 22F with some decent winds last Jan. Neither of us had bothered to put on the overcover at that point...just didn't seem cold enough for it yet.
I suspect we could go a good bit colder if we had to by putting the overcover on, and doubling up on some of our insulation inside the hammock (I could use my fleece liner inside my 20deg mummy bag, placed inside my 15deg military bag, with a pad underneath that inside the hammock. Throw in my poncho liner and the full HHSS...I'm figuring I should be good to some pretty low temps if needed.