Down in a synthetic bag?

Tommy,
I wouldn't take this approach. The baffles are engineered around a calculated amount of down, and if you stuff more in it will interfere with the extent to which the down can loft and hence less warmth. Stuffing down in a synthetic bag would be even worse as the down would get all balled up in the batting and not loft at all. If what you are looking for is to extend the range of your new bag, then consider pairing it with a bivy bag or a vapor liner or both. Happy trails!

Thanks for the tip. I know after reading on here that the slumberjack bags don't really sleep the rating they claim, but it was a gift and I thought I would try something.
I ended up buying the SJ Super guide 30 degree bag and use them together. It was still light weight for me…taking my base load from 11 to 13 lbs, and I only need it during winter. otherwise I only have to carry one of the bags.

I am new to light weight packing,(I don't miss the weight at all) and have now gotten my gear down to 11lbs. base for spring through fall and 13lbs. for winter and I am able to fit everything into a Eureka Getaway 2500.

Tommy