I've been very happy with the 7D (or is it 8D) fabric of my quilt (M55). I think sleeping bags/quilts are the perfect application for these very light nylons. With reasonable care, I have zero concerns that I'll ever damage my M55 shell. Keep in mind people are using windshirts made of similar stuff (ie. Montbell Tachyon Anorak) and trudging around the woods wearing it......my sleeping bag only comes out in the safety of my tent. Where heavier fabrics come in is if you have a sleeping bag with a zipper....then a zipper can cut these lighter fabrics. Also if you're using low grade down with huge feathers in it then maybe they would poke through these fabrics but it's really a red herring because no one would put crappy down in a sleeping quilt with high end shell fabrics.
For sure, more down leaked out of my Ultra 20 (with 15D or 20D) fabrics than my newer quilt. The Ultra 20 was still good and it was a non-issue, but every month or two you'd see a feather poking out. It could have more to do with the lower quality down (ie. more feathers in the down) than the fabrics though. True down doesn't have a stem, so it should never poke thru even crappy fabrics.
Some of these really light fabrics aren't that 'downproof', but it depends more on how the fabric is made. It's not as simple as lighter nylon = less downproof. M55 (and then new M50 I believe) are unbelievably downproof. Whereas the original M50 wasn't that downproof because of the way it was made. My understanding is that M50, 7D and 8D are all very downproof and it's an complete non-issue if you're using decent (800+) down.
My M55 shell fabric sounds like it acts a lot like Tim's 20D in the sense that it's so windproof that it can be a little 'hard to pack'. You almost need to squeeze the air out of it like you're squeezing a sleeping pad. It also takes a little longer to loft up, not because of the down but because it takes a while to suck in air through the fabric....not a big deal at all and much appreciated for the bomber downproofness. So really any nylon can be totally downproof if it's made right.
I wouldn't look at synthetics in your situation.....there's a lot of cons (lifespan, weight, pack-ability) and keeping a down quilt dry in your circumstances shouldn't be hard. Using a little of the weight you save to get a cuben roll top sack to ensure there's zero chance it gets wet in your pack.
Regarding 900fp down and possibly sucking in humid conditions, here's a few quick comments:
- I believe RJ said it may not be better than 800fp down in humid conditions (because it won't really loft up to 900 in anything but perfect conditions). I don't think he was saying that it was a bad choice...just that you might not get the benefit of the full 900 and it would perhaps just act like 800.
- It seems that RJ's comments are based on the idea that 900fp down uses down clusters that are more awesome/higher quality and extra lofty vs. 800fp down clusters. He then argues that these's 'extra lofty' down clusters won't actually give you that bonus loft in non-perfect conditions, so perhaps they're not worth paying extra for vs. 800fp down. This may sound reasonable, but I don't think it really reflects reality....at least not my experiences with 800/900 down. In my experience, the main difference between 800 and 900fp down is that there are fewer feathers (ie. virtually none) in the 900fp stuff, whereas 800fp down is commonly about 10% feathers.....which aren't very thermally efficient and they like to poke holes in your sleeping bag. So the actual down clusters in 800 and 900fp down are basically the same IMO, there's just fewer feathers in 900fp down which is a good thing for many reasons.