I thought the same thing for a really long time, when I was still in the midst of switching to hammocks / still doing ground camping I even invested in a 0F Kelty bag... It was the first time I was ever warm the entire night through, but every time I'd use it I was far too warm, and would always wake up sweating.
I recently had the pleasure of getting a 20F + 2oz overstuff Warbonnet Diamondback, and man, the cottage top quilts are something to behold. Extremely light weight and compressible, and it really retains heat. I have only had an overheating problem once, and that was when the temps for the night dipped up into the high 40s instead of the low 40s high 30s that I was anticipating. I'll probably get another TQ rated at 40F some time next year, and I'm excited to see how low I can go with the Diamondback, but I am very confident that it will keep me very warm.
If you're worried about staying as warm as possible, don't forget to have the footbox sewn instead of zippered. I have a zippered footbox because I often sleep too hot and I wanted to have the ability to use the quilt more as a blanket. As KBr00ks said, the temp ratings on the big retailers are veeeeerrry different than the cottage vendors. I've slept in 40F-20F bags in 50F-30F temps respectively, and was extremely cold! But the 20F TQ I've used at the lowest 32F and I was extremely comfortable.
2¢!
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