I'd skip the GT shell if you're planning on using it anywhere near freezing and above. It's helpful in cold, dry conditions but won't breathe as well as a standard shell in warmer, humid conditions, possibly compromising the fill.

I have two WM bags (Ultralite, Dakota) and they're great--highly recommended. I also like Feathered Friends. Look at the Raven and the Swift (possibly with overfill) as alternatives. Every bit as good as WM bags, and much more customizable.