After much research, being somebody who freezes easily, I replaced my long-serving (10 yrs plus!) Thermarest LE (1050grammes-40oz R.4.1) with a Women's Pro Lite 4, extra-long (750grammes-27oz R.4.1). This was the warmest I could get for the weight.
While the Pro Lite 4 is a tapered design, it will fit in my bivi-sack, whereas the square LE will not. You may want the bulk and size of a square pad if you roll around a lot when you sleep, as tapered-foot pads let your feet fall off more easily, then they freeze!
In high humidity (80% and up) winter camping at -10C/8F on snow I use an aluminized space-blanket tarp under the tent to keep body heat from freezing the tent into the snow, a Bibler I-tent, an 18mm purple Evazote foam pad (full length) and the full-length LE. Then a -5C down bag, recently cleaned by Kluane Mountaineering after 18 years of service, with a bit of new down added, down pants, down jacket, balaclava, pile long underwear, contact gloves and insulated hut booties from MEC. This takes me to about -15C/+5F. I look at the Pro Lite 4 women's XL size (in hi visibility pink!) for summer-fall use and then will experiment with it in my back yard before using it in the winter! Taking a pee bottle with you also saves a trip outside!
This 'spring' (-15C/+5F) am going to experiment with a vapour barrier liner made of orange garbage bags, and also some of the Stephenson Warmlite VB undies.