"I am aware of the EN rating system, and how it is often an overestimate of warmth."

I have a EN rated bag and it is spot on as far as I am concerned. If there is any abuse of the rating system, it is by manufacturers who deliberately mislead customers by only posting the extreme rating. Here is how ratings should be presented-
Temp rating Comfort -11ºC / Limit -19ºC / Extreme -40ºC
This is from a bag on the MacPac website. I have one of their bags (not this one).

If you don't see the complete rating, I wouldn't trust what I read.

Valandre, a lesser known but very high quality French bag maker publishes a complete report on its website for each of its bags.
www.valandre.com
Here's an example, click on the PDF link to see the report.
http://www.valandre.com/eng/Sleeping_Bags_and_Outerwear/sleeping_bags/shockingblue.html