As mentioned, the manufacturer's temperature rating means very little, and it's best to check the EU ratings, if the bag is sold in Europe as well as the US.
My own experience with a Marmot Hydrogen (30* F), is that I started having to add clothing layers in the upper 30's. On a 26* night, with all my outer clothing (even rain gear) on inside the bag, I still was shivering. Clothing included midweight base layer (REI MTS) top and bottoms, nylon hiking shirt and pants, 200 weight fleece jacket with hood, fleece cap, polypro glove liners, frogg toggs jacket and pants (with hood on), heavy acrylic hiking socks. Sleeping pad was a Thermarest LE. Where I was coldest was on my hips, even though the sleeping bag was not stretched tight (I checked). The dog slept at my feet, so they stayed warm. I slept on and off, but kept waking up shivering. I needed two more dogs--one on each side of my hips!
Last August, with temps down to 20*F, I was fine in my Western Mountaineering Ultralight Super bag, with base layer plus nonbreathable rain gear (Brawny Gear rain jacket and pants, now sold by BPL) as a vapor barrier liner. I also wore a polypro fleece balaclava and a pair of 200-weight fleece socks (much more comfortable than hiking socks). The draft collar in the WM bag really made the difference--once I snugged that around my shoulders, I was just fine.
Both nights were windless, BTW, and I was in a single-wall, well-ventilated shelter both nights.
Obviously, I'm a cold sleeper. If you are one also, don't plan to get that 40* bag below the mid-30's.