some companies (Mountain Hardware) are reputed to do better than rating

More than likely is that you are thinking of Western Mountaineering, rather than Mountain Hardwear. WM's reputation for excellent sleeping bags is pure gold.

If you sleep cold, then REI bags prior to their switch to the European warmth ratings are all I can speak to. They tended to be a bit less warm than their ratings, but not too badly. For instance, I have slept in their SubKilo (rated +20) in temps quite near to +20 (w/ hard frost inside the tent in the morning), and I felt warm enough, but I was wearing a full suit of 100 weight fleece inside the bag.