I recently upgraded my sleeping bag from an old 20F Kelty to a lighter 10F Rab. I slept out in my backyard around 15F and was fairly comfortable with base layers, although my legs got a bit chilled as morning arrived. The weak link in my sleep system is now probably a my 3.3 R value REI stratus pad. I'm thinking of upgrading to a NeoAir XLite, which seems to be a well-reviewed and popular bag. Supposedly the R value is 4.2, and it's a good half pound lighter than my current pad. I'm curious how others have fared with this pad at lower temps - say, below 20 degrees. (From what I have read, the temp ratings on sleeping bags assume a pad R value of something like 4.7.) Should I spend the $ and extra ounces on an even warmer pad? All stories and advice appreciated.