I have not used or even handled the syn mat but, based on my experience with the UL7 down mat, I am surprised by peoples' need to use the syn mat with a foam pad to ensure warmth. Based on my experience I would have thought the R value difference between syn and down to be less obvious at those temperatures, i.e above freezing.
These are anecdotal examples but illustrative of the effective R value of the UL7 down mat for me:
1. I have slept in -24C (-11F) on a UL7 LW downmat, inside a 3 season tent (BS Evolution, with only the sylnylon floor between the pad and snow, ie. no foam pad. I wore 2 merino base layers (150 grams each), a toque and fleece neck warmer. I was in a WM Ultralite bag (rated to -7C\20F) covered with EE Revelation 20F (rated -7C\20F), cinched tight and I was toasty warm.
2. Last year while sleeping on a rocky river bed (rounded boulders cemented in place by compact sand) my down mat kept deflating and after pumping it up twice early in the night, and then assuming I had a leak, I managed to find a bearable sleep position and left it deflated and slept on it without air for about 6 hours. The temperature that night got down to -1C (30F). Again, I was toasty warm. Used same bag as above but no quilt.
The next night, the last night of the trip, assuming it had a puncture and because I was sleeping on a sandy beach which was acceptably comfortable, I didn't use my mat and slept with only my sleeping bag (same bag as above) and the silnylon tent floor between me and the ground. That night the temperature went down to 1C (34F) and I was cold the whole damn night, to such a degree that I mummified the bag to the fullest extent possible and maintained a fetal position to stay warm.
Conclusion, the down in the deflated mat provided considerable insulation even without air in the chambers.
Maybe the down mats are a whole lot warmer than the syn mats. I am unsure how R values work. Perhaps the increase in value has exponential effect.