This system may not work for you, but it is my 3 season system; and I only give to offer a suggestion on sleep systems and bags.
A: I love my WM highlite (35f...18oz)
B: I already carry a patigonia silk weight top and bottom for those frosty morning and evenings when it dips below the 40F mark (which is the norm for my hiking region and elevation of hikes) and a Montbell U.L. down inner jacket. This is my 3 season kit.
C: I allready have all of that 'cold' weather gear why would I want to carry a bag that is rated at 15-20F and sleep in it with just my skivy's, and have to carry the extra weight of the bag plus the weight of my clothing? So If I know it is going to get "cold" (according to me) I will put on top, or bottom or both, depending. When the temp really drops on goes the Ul jacket and beanie hat. Or if I get cold in the middle of the night I will put on what ever layer is appropriate.
D: This ultimately saves weight which equates to hiking comfort and yet there is a large enough safety margin to where one can stay warm.
E: Like the others you get what you pay for. I love my WM and I am a 200lb 73 in broad shoulder guy myself. The bags are straight forward and are no frills in ragaurd to lots of zippers, pockets, exotic designs and colors etc, that alot of the manufactueres use to catch a customer. And like others have said as well, when taken care of they will last a life time.