I can't seem to find any info from the manufacturers site - I have some bags that compress down fairly well into their compression sacks, but can compress further with a compression sack with belts on it. The question is, will this hurt the down (for short times, during expedition of course - normally stored expanded. I don't want to damage the bag permanently.
It'll be absolutely fine for trips.
Lots of comments online of people who say it will damage it with no proof, but a few saying they've called manufacturers and asked them and they've said it's fine so long as it's dry. Considering the cost I really want to make sure I don't cock it up!
IME (30yrs+) it has never damaged the down on various bags I've used; based on daily compression for trips of up to 2 weeks and then airing and storing uncompressed at home. As long as bags have been allowed to air and dry properly after trips, the down has always lofted fully.
As long as you don't normally store it like that you'll be fine.
I've always called the bags without belts 'stuff sacks' and the ones with belts 'compression sacks'. Most down bags come with proper compression sacks as standard which suggests the manufacturers are ok with it! The only bags that can be stored compressed for long periods without damage are Buffalo ones but they are huge and heavy for their warmth.