You could use it, but it wouldn't be as light, compressible, or durable as a high quality down. I would guess that, at best, it's 500-550 fill power. A high quality down (800+ fill power) should have minimal quills and feathers. The quills in a lower fill power will puncture a lightweight nylon shell, and they may poke you uncomfortably as well as damage the shell.
Separating the feathers from the down would be incredibly tedious and time-consuming. Down likes to fly away, and it difficult enough to deal with when you're just dumping it from a baggie to a baffle. I really wouldn't recommend it.
At 850 g/m2, your quilt won't be very light. For my non-metric mind, that's 24 oz/yd2. That's more than my homemade quilt weighs altogether, and you'd still have at least 8 oz of shell fabric and baffles.
What you could do, since I'm assuming you're on a budget, is find a moderately lightweight down bag on ebay that you don't mind cannibalizing. I recently bought a GoLite Feather-Lite bag for $86 US that I'm going to turn into a quilt for my mom. There are instructions on this site, if you haven't seen them.
If you're determined to do this, just be aware that you will have a relatively heavy quilt, and you'll need a heavier shell fabric to protect you from quills. Down-proof might not be feather-proof!