i have a caribou, and i'd be careful thinking it's good to 25 with just some long johns on. like all WM bags, it's well rated (35), and probably a little conservative.
i have slept in it in a bivy at about 30 with long johns and a puff vest, and i was not cold, but not super comfortable. there was probably some effect from cold ground (and thin pad) and wind.
it's a fantastic summer and shoulder-season bag, but if i was going to be down to 20, i'd just take my Alpinlite instead. i don't think it makes sense to carry insulated tops and bottoms that are too hot to hike in, just to stretch a bag down another 15 degrees. (of course, WM bags aren't cheap!)