**EDIT** Just noticed you said you've airway read my MYOG thread and others! Sorry! Hope I wasn't coming off as rude or whatnot! I was just too lazy to go find my link (all this is being written on my phone)
From my experience, as a warm sleeper, I was perfectly warm at freezing with just a single layer of 5.0 Apex (from thru hiker), Synmat UL7, and just 200wt wool base layers and 100wt fleece beanie. I was in a Tarptent Double Rainbow, with ~30 mph winds whipping by all night.
I know I can take it into the twenty's by throwing on the layers/puffies I bring with me at those temps. I think this gives the best versatility/range of temps, rather than having a bulky synthetic quilt rated for 20-30, have a quilt rated 30-40 and supplement with clothes that'll be worn around camp for warmth.
And yes, you only need to sew the perimeter. No need to stabilize like you would for Primaloft One, etc.
My quilt with straps comes in at 24oz.
If you're not as warm bodied, I'd either get the 6.0 from OWF or do a 5.0 & a layer of 2 or 2.5 over just the torso or the whole thing. But 7.5 total will be quite bulky.
I mentioned in my post that I didn't add anything to finish closing the bottom other than tying the cord, but I have since added a few snaps along the last 12" or so to keep it closed better. If I were to do it again, I'd use a ~18-24" separating zipper at the bottom/legs.
The quilt strap system really is the bee's knees. I have a video link posted somewhere here in the forum, I think, of it in action. I can dig up the youtube link for you if its hard to find. You can also search "quilt strap system" or the like on youtube and should be able to see me rolling around in it.
As far as fabric goes, since you'll be tarp camping, maybe something with some good DWR on the outside fabric, and softer/cheaper on the inside, like some 2nds ripstop.