Thanks guys. Your experience seems to validate the claims. Sounds like the liner + 40 or 45* bag is the answer. Then in effect I'd have a 10* bag (my existing 20* bag + a liner), a 20* bag (existing bag alone), a 32* bag (new 45* bag + liner), a 45* bag (new bag), and a 60* bag (just the liner). Lots of options
I do not do self-supported touring so my gear list is a bit, umm, luxurious. Most of my touring is in Idahho where a week's traveling can take us from desert at 1500 feet to 9,000 feet alpine canyons in a day. Nighttime temps on the desert might be 60-75F but 25-35F the next. So have learned to I carry two down bags from Marmot, a one-pounder and a two-pounder. I also carry a silk liner. This gives me a wide range of comfort levels and the silk liner has kept the down bags quite clean.
If I were carrying my own gear, I'd go for the 2-pound bag, a silk liner and a set of silk sleepwear.
BTW: Down is queen in Idaho's climate. If I'm doing Cycle Oregon, I know we'll see rain on at least two days. I take the one-pound down bag, a 2-pound synthetic and the silk. The down bag is double-bagged in waterproofness and compresses to the size of football.