Well, I hesitated to post my comments about the North Twin, which I've owned for several months but have had only a handful of instructive experiences with so far. You can get a sense of my learning curve with it in my winter q's thread
I think the Microlight shell is breathable enough to pass moisture from wet clothing quite well. Warmth:wt ratio is a trickier matter though. Since I had mine made in 'broad' shoulder girth, to accomodate belay parka and pants ~inside~ the bag, really I could have gotten a much colder rated but tighter fitting bag from TNF for same wt. I wouldn't expect a wet parka to dry well when draped over the bag, as opposed to worn inside, but strictly speaking the latter may be more efficient, moisture management issues aside.