I have a Kelty 20º 800 fill down bag that has a comfort rating of 32º. For me, those EN13537 ratings on that bag are very accurate and it's the first bag I've had I can say that about, so I'll blabber on a bit about it...
I've spent a lot of 30º-40º nights in it and have been very comfortable in just long johns and a light shirt. I've spent quite a few 20º nights in it too, and for those I got chilly until I layered up.
The coldest nights I've spent were below 20º and I put my down jacket over the bag. That kept me comfortable to probably down near 15º. I've also used one of those emergency blankets over it on cold nights and they work very good to add a few degrees of warmth too. I did get a thin layer of frost on my bag doing once that, but it never got wet and I just brushed it off in the morning when I found it. With that and my jacket and layered up with everything I brought I was probably good to near 0º. I know I was warm while everyone else with me wasn't, but they didn't have down bags or jackets or SOL blankets.
I've also spent a few 50º or so nights in it, and mine isn't easy to vent so I didn't stay so comfortable on those warm nights. My bag has a 3/4 length single zipper and I was sweating a bit, or chilled. I really didn't like sweating in my down bag so that alone kept me up at night wondering if I was going to stink it up forever. So far I haven't, but I lost sleep over it.
I don't see how the ratings on a WM bag should be any different. The weight, materials, design and craftsmanship may be, but not the warmth you'll get out of it.
In my experience an EN13537 20º rated bag works perfect for the temps you're wanting it for. If venting it is something you'll be wanting as a feature I'd look into designs that offer that and if WM has that, great, if not, I might look around some more before clicking the buy button.
In my case, I was looking for value and comfort at 20º-40º, and the Kelty I got was a very good one when I got it. I really shouldn't expect it to be comfortable at +45º, or even +40º considering the design, and I was thrilled at being able to make it work at below 20º. The biggest downside is it's a bit tight, and I'm not a big guy. I'm about 5'9 and 140 with a full belly.
I have to think WM bags in the same range would be impressively better.
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