I went as far as pangbouche on the Everest base camp trek on my way to Ama Dablam. The tea houses were quite warm in my 15oz 35deg western mountaineering highlite bag. This was in late Oct. No need for a thermarest unless you are camping.
As an aside, my prolite 4 held up all the way on Cho Oyu despite all the warning that it was going to pop. I'll bring a ridgerest just for backup but that thermarest did wonders for a soft sleep on bumpy ground.