The gumby suit is indeed very warm. And yes, you bring the sleeves into the suit for a toasty sleep, so tossing and turning is no issue at all.
I used mine one snowy night in PA. I was on an elevated wood floor in a shelter (so plenty of cold air circulating under the floor). I was in an MLD bivy on a NeoAir and Ridgerest. The temps got down to 28 degrees.
I started inside my Raku (the gumby suit) wearing Icebreaker 200 briefs and shirt, RAB vapour rise pants, Nunatak Skaha down jacket, and some type of beanie. And my wonderful down booties. Within half an hour I removed the down jacket and vapour rise pants because I was too warm, unzipped the bivy and stuck my head outside the bivy. I slept that way until about 2 a.m., when I zipped the bivy mostly shut, but didn't put any clothes back on. I stayed warm the entire evening, and even during breakfast as I kept the Raku on for that.
It's a great, albeit heavy/bulky, piece of kit!