More nitro does make an engine run easier in cold weather, and in many engines it will improve idle and trasition. I belive the nitro makes the fuel burn just a little hotter, but I'm not sure.

The theory about cold, dry air having more oxygen makes sense, but the real world experience of many many years of many many guys doing some very cold weather flying has shown that adding nitro helps.

GA just doesn't get cold enough to make a difference, you can use 10% if you want, no problems. It's only when you start to try flying in the snow that it matters more.