Double the weight. Four times as much space. Not automatic deal breakers. But easily apparent disadvantages. If everything on your bike trip weighed twice as much and took four times as much space to carry, you would not be able to tour by bike. Unless you wanted full panniers and pull a BoB. And only rode 30 miles a day. Would you tour with a large 8 pound tent? Most bike tourers probably try to find a small 3-4 pound tent. Would you tour with biking shoes, hiking boots, sneakers, and thongs? Or just use your biking sandals for everything or maybe also include a set of sneakers with the biking sandals."
Washing rather than dry cleaning is recommended. Although down bags can be dry cleaned without damage, only a dry cleaner with extensive experience dry cleaing down is likely to do it correctly.
As I suspected from your prior post you haven't been keeping up with synthetic fills. My experience is much different from your comparison of your down bag and catalog description of synthetic bag. My down bag weighed nearly twice my synthetic bag; I keep just as warm in the synthetic bag and bulk is less. While volume is a consideration, I wouldn't place undue emphasis on it for a down bag. Over compression may harm the down, and tight compression can significantly retard the down's restoration to original loft. I would expect in time more original loft is lost in a down bag than synthetic.