I bivi’d in the hills between Coulags and Torridon and Lochan Fada earlier in the year. Both trips around 6c I was sweating like a pig with cycle shorts/merino base layer in a down bag (suitable for down to around -5c)
I would guess that without a self inflating mat (I had a Vango lightweight mat that inflates to around 30mm thick) the chill from the ground would go straight through the bivi and sleeping bag and you’d soon be wrestling around to keep warm by sliding back onto the mat (also can be a bit tricky when your arms are by your side and you are on off camber ground!)
For my next bivi I plan to make a trough to lay down in.There’s a few small pebbled beaches around Lochan Fada that will hold the inflatable mat well. This should spread the body weight out well without feeling like I’m sleeping on three points of my body at any given time! (like what happened by choosing the wrong spots next to rocks on exposed mountains with rocks digging into my back :O/)
ALso go for a full length mat, your feet will still freeze hanging off the end of a short mat,with merino socks,a down bag and bivi bag between you and the ground. Its bad enough in summer.
Edit: Use ear plugs for a silent night. The stars,sattelites,meteors etc are amazing to watch before crashing.Upon waking there’s no midges (If you plan on coming up here), and the fresh air will mean one of the best nights sleep you’ve had in years – possibly a lifetime so it’ll all be worth it.