2.5 Apex is what I would use.
Do NOT, however, use Silnylon. It will exasperate precisely what you are trying to avoid - a wet down bag. Use M50, Nobul, M90, 1.1, or other light, non waterproof fabric.
Since the sil is waterproof the moisture will stop where sil touches your down bag. You know how that will end up.
As for the noise - I've used a "topbag" over my quilt and didn't notice any noise at all aside from the slight brushing sound when I roll over. Shouldn't be an issue.
Please show us what you end up with!