I camp mostly in the ADK's of NY. Temps run from about -25F to about 100F over the couse of a year, with some extreme days being worse.
My tarp does well down to about 25F, stopping most breezes/winds and convective heat loss. My 4 season tent is an Exped Sirius. At 30F it maintains about a 10-15F temperture differential between inside and outside air. Yes, we get some condensation on the outer fly.
Generally speaking, an open sky is quite cold. I have heard of reports down to -70C or more. Mostly this is due to radiant heat loss, though. General convective loss is helped by stopping most winds/breezes. So, taking a 40F bag down to 30F is OK in a small tightly enclosed tarp shelter. You will notice the difference when you exit the tarp in the morning. Condensation is always a problem in such shelters, though.
A double walled tent, such as the Sirius, has an inner breathable liner and is good to about 20F with a 40F bag. This presupposes a good pad. (I use a nightlite and neoair in combination or about an R4.7 or a bit higher due to lofting in the bumps where I don't compress them.)
I have used this tent in June (late spring/early summer) but found that it was too warm for comfort. I always have both vents wide open, so, this is not a summer tent. Mainly I use this for base camping/car camping due to the ~6pounds of weight.
The tarp I use is larger and weighs a bit more than 17oz. It allows me and my partner to sleep in an A-Frame type setup, or, lean-to depending on the temp. A Diamond setup is sort-of a compromise, so there are several pitching options availble. July and August, I often bring a mesh tent for bugs with a partner. It weighs about 16oz. There are no notable condensation issues, except the tightly enclosed A-Frame pitches. All waterproof layes will have some condensation under some conditions...
Basically, I would consider my use, be it summer, spring/fall or winter and purchase a tent with a warmth factor fugured in accordingly. BTW, I have spent several years camping, so have refined my requirements beyond the average, I believe. But, as always, they are MY requirements, not yours.