Can't think of his name, but there is a guy over on BPL's forums who makes custom bags - and kid ones he does as well.

With my oldest, Ford, I didn't do down though. He had a Tigger by North Face, which while a bit heavier (1 14?) it was a well built bag that took a lot of abuse and still looked new when I sold it when he got too tall.

Only problem with a sleep sack would be arms getting cold at night. I understand the sleep sack though - my youngest, Walker, used one between 3 and 6 months.