I have never been warm in a synthetic bag. Once I went all out and bough the most expensive, lowest rating, man-made fill bag at REI. It was supposedly rated to something like -40F, don't remember. The point is, I froze my *ss off in a trailer with no heat at only about -15F. And I was wearing all the clothes I had with me!
Ever since then I only carry down bags. If a rain is expected, I simply pack it in a large bag and stuff it deep inside my backpack. Never had any problems this way.