Silk isn't calendared like downproof nylon so most likely not useable. You can always make a pillow to test out (under fill it, overfilling a pillow stabilizes the down enough to not make a decent test).
As for sewn through baffles. The general wisdom is that due to cold spots they're only good to around 35dF. Sure the thicker you make it, those thick portions will be toasty but the seams will still be frigid. It's a weird tiger striping sensation of temperature.
So unless you make the baffles so big that they kind of form baffles anyway then 2" is probably the max usefuleness and anymore would just be dead weight. Of course YMMV with how you deal with cold spots.