compressing a down bag

I just bought a kelty cosmic 0 deg down bag and the manufacturer's stuff sack is not compact enough to my liking. Does anyone have experience buying an aftermarket compression sack, cranking it down on a down bag farther than the manufacturer reccomends (I am assuming they choose their stuff sack sizes for certain reasons)?

Does/did compressing the bag as small as it can go mess with the fill? Were there cold spots? Over time?

Thank you for your time.

I think the risk is that down has only a limited ability to recover. And the quill might brake. At the end you might loose some insolation properties.

rgds, Peter

That bag is only a 550 fill. The 550 and 650 are not very compressible. The 850 is really what you want for compressibility, but they come at a price. The 850 is what most use on expeditions, when size matters in the backpack. I was lucky to find one on steep and cheap for less than $200. The downside to the 850 is when any body part pushes outward in the bag, it easily pushes the down away and creates cold spots, even on a zero degree. So I don't think it is the compression sack, but the fact that that fill doesn't compress well. I wouldn't worry too much about losing temperature ratings after compressing that bag, because there is so much down in those bags it will easily recover. Just don't store it stuffed.

For bikepacking I smash my Mountain Hardware bag down all the time. And yes it's lost some of it's loft. No cold spots per say but I can tell it's not as warm as it used to be.

6 years old now I always keep it in the large bag when not in use. It's still works good enough. Needs a new zipper soon tho.

You just have to ask yourself how long you want it to last vs how compact do you want it to be for travel.

I'd just get a smaller stuff sack/dry bag. Trying to go smaller than you can stuff it by hand is a bad idea imho. Trying to go to small is really an excersize in futility; the compacted bag is so rigid that it will do a poor job conforming to the shape of your pack and all the other gear around it, leaving inefficient voids. My preferred way to pack is with relatively loose stuff sacks for organization, and then treat the pack bag as one giant stuff sack.