Over compressing any bag, whether it be down or synthetic, will eventually lead to loft degradation. If you compress your bag too tight you can cause damage to the barbs and barbules of the feathers, which will decrease loft over time, but this is apparently less of an issue with higher quality down. To be honest, I think you would have to have one incredibly hefty compression bag to actually compress a sleeping bag down to it's absolute minimum volume. Every compression bag I've ever owned seems to have a limit, usually where the buckles start to slip, or the seams start to burst. Never leave your sleeping bag compressed for longer than you need to.
Personally, I don't use a compression sack for my sleeping bag anymore. I use the roll top drybags with the webbing clips that came with my newer down sleeping bags and stuff those into the bottom of my backpack. They compress the sleeping bag more than enough for it to fit nicely in the bottom of my backpack, and they help keep it dry. Putting soft items in an overly-compressed compression sack turns them into hard items, which don't fill you bag very nicely. I've found that I can actually save space in my bag by allowing the soft sleeping bag to fill the bottom of my backpack and packing it down, opposed to putting a solid compression-sack in the bottom of my backpack and then trying to fill in all the voids around it.
I'm not the only one that prefers using drybags for my sleeping bag: The Experts View: Expedition Sleeping Bags