My take is that you're right at the edge of where down makes sense. The difficulty of getting a small amount of down well spread-out without dead spots either results in over-stuffing or very complex baffling, both of which add weight and cancel the inherent efficiency of down.
I've run the numbers for a few different quilt designs, and have found very minimal weight savings for a down quilt until you get to the 20's, which is generally agreed to be about 2" of down or 5oz of Climashield XP.
You would be looking at 1.5" of down or 3.7oz Climashield Combat, where you'll be lucky to save 1oz and avoid dead spots.
Sewn-through is probably lighter than baffles for something of that rating. It's certainly easier, and you can just over-stuff instead of spending weight on baffles.
So - sew-through and over-fill, or go synthetic. But run the numbers first to make sure the down will save you weight before you spend the time and money on it.