I totally agree with your assessment of temperature ratings WD. I have never slept in a bag down to it's rating, although I have gotten close in my WM bags. It's a very subjective thing based on the person and conditions. For example, I can sleep in my WM Caribou almost to the rating if I am in my Akto tent, but I've gotten cold in my WM Ultralite at 40 F in my Double Rainbow.

To the OP, I have 3 WM bags and I love all of them. I do have an Ultralite, and I am not sure if I am going to keep this guy for the long term or not. The main reason being the amount of room inside of it. The cut on it is fairly restrictive, and I am finding that I prefer a bag to be a little roomier like my WM Caribou. So definitely try it out first, and see if you like the cut.