I used the Hobbs washable wool batting for one of ny quilts, it's probably the warmest of all my quilts so far, but I've never used Poly Down so couldn't tell you if it's warmer than that!
I hand quilted it, so can't give advice on machine quilting with it...it was lovely to hand quilt with though, and after it was washed it became quite puffy and crinkly!
It's one of my favourite quilts to snuggle under!