Belated update time. I decided to buy the Thermarest
Boost bag, the -6C / 20F version.
Tested it on a 3-week camping holiday, in coastal BC, Canada, in May. There were plenty of chilly nights, as well as the occasional warm one.
The bag was great. The additional space you can get by opening the central longitudinal zipper really makes it comfortable, especially for a side sleeper like me. On a couple of cold nights I zipped that one up for extra warmth, which worked a treat. The armholes are useful either for extra ventilation when it's warm, or to let you rummage for your torch/phone/glasses without getting out of the bag.
I particularly liked that I could get a short bag, it's just big enough for little 168cm/5'6 me, and I'm not carrying round an extra 6" of sleeping bag that my toes will never reach the end of!
It packs down much smaller than my previous 3-season synthetic bag, it's just as warm, and it's more spacious. Recommended.