I can't say I have extensive experience sleeping in the Serape, just one test so far.
I was in a Tarptent Notch li with "solid" walls (dyneema floor with ripstop nylon sides and mesh top interior.
I used the Serape with a "thin" poncho liner on top of a Thermorest Exolite plus.
Temp got down to about 25 degrees that night. I was ok, but I sleep warm and can tolerate cooler temps than most. I would say it's good to 40ish give or take 10 degrees F for most people.
I think the smaller tent had an effect of keeping a micro climate as well. I wouldn't push it much past where I was though, I was curled up pretty tight.
Side note, that Exolite Plus pad is the shiznitz.