Good Morning G2S,
I too have an Exped Synmat. But I was using it alone. I was warm enough on the bottom though.
I did run into someone who had a +20 bag with a bivy sack and he said he was plenty warm at trail camp.
the part that I left out was that there was a part of my lower back that wasn't covered by the sweater jacket that I was wearing.....that was the coldest part of my body, even colder than my toes. And the inside of my bag felt cold to the touch wear the bag was closest to the sidewall of the tent.
