Believe it or not, Western Mountaineering bags do have EN13537 ratings, but WM doesn't publicize them. Their bags are sold in Europe, where the ratings are required, but it may be that the European wholesalers, rather than WM, have the testing done.

Several years ago, I looked up my bag, the WM Ultralite, on a UK website. Unfortunately I didn't bookmark the site so I can't document this without searching all over again. The comfort rating (for female cold sleepers like me) is 24*F. (I used a conversion table since of course the UK site used degrees Celcius.) The lower limit (for male warm sleepers) is 17*F. I suspect that's pretty accurate, since I've found I have to add insulating clothing at around 24*F . With all my insulating clothing on, a vapor barrier suit underneath (but over my base layer) and a warm enough sleeping pad, I've been cozy at 15*F. As mentioned, I'm female and a very cold sleeper.

For the conditions you describe, the Versalite would be overkill. You always have the option of adding insulating clothing layers if it gets colder. That's your safety margin!

Do remember that the EN13537 testing conditions have the dummy wearing a base layer and a knit cap. For testing 20*F bags the sleeping pad has an R rating of 5.