Bag "ratings" are a tricky game. Many manufacturers have begun rating their bags according to the EN13537 standard. REI and Marmot, for example, are doing this. However, not all of REI's bags are rated to this standard. The Kilo Expedition, for instance, came out before they moved to that rating system. Also, a women's bag rated to 0 is going to be significantly warmer/heavier than a men's bag rated to the 0 on the EN system. That is because they rate Men's bags at the "Lower Limit" number while they rate women's bags at the "Comfort" number... whatever... Just know what you're looking at.
Personally, I do love my KiloExpedition but it has a double zipper that can be a bit annoying sometimes. Also, with the second inner zipper zipped up for maximum warmth, it's a bit tighter inside than the specification would suggest.
If you need much warmer than this bag, then you are definitely looking at a "-40" bag from MHW or FF or the the like. Keep in mind, you can always just decide to wear puffys to bed and get away with a lot lighter bag.