I'd consider spending a little more and getting a 20 degree down mummy bag that'll be lighter and stuff much smaller than the 40 degree model you're considering. That's what I did 35 years ago when I got my current bag and it's been used on quite a few trips every year since then. And, unlike you, I'm not yet in the market for a replacement.
Mine has yet to get wet except when being washed, but bike campers are usually not in as bad a predicament as backpackers if their sleeping bag should happen to get wet. We're usually not as far removed from laundromats and locations where emergency shelter could be found. And I find the compact stuffed size of down bags to be particularly useful for the bike camper.