Good/bad effects of turning 4"wide into 8"wide down tubes on a sewn thru quilt?

I don’t think that other things being equal, that a sewn-thru quilt will produce anywhere near the same insulative value as a bag or quilt with baffles that are not sewn-thru. Should you think the weight of a 3/4 length zipper, less than a half oz total for four feet in #3, is prohibitive, I would still opt for a bag with baffles that are not sewn-thru.  (YKK #3 coil zip weighs 0.1 oz per running foot)

Ideally, an Apex bat would be large enough to only need to be sewn thru around the perimeter of the quilt. If not, I would put baffles that are not sewn-thru between batts that are cut as large as possible. An interesting baffle material to try might be that used in stretch panty-hose, as its high stretch would not interfere with the quilt or bag draping around the body, and would be less likely to tear..

Some may be leery of sewing baffles between two shells. But in my experience it is far easier than sewing a tent, for example. Preparation and marking seam lines seem to be the key. And I’ve always sewn in a small flap at the seam lines because it makes it easier to sew on the baffles, and eliminates exposed thread on the outside of the shells. Here’s an example of a single pattern piece for one of two shells, both flat, and after sewing to shape:

Note that an additional flat piece must still be sewn over the head area, with a flap created for a draw cord to tighten the head cover..  The design is for a center zip, and probably needs more width at the feet.