You might consider a down quilt from a cottage builder, like Enlightened or Zpacks.
As for warmth, if you're using very high quality down (850-900FP), then it is much more logical to add a few ounces of down to a quilt/bag, rather than bringing/wearing extra/heavier non-loft clothing to bed in order achieve a lower effective temperature rating.
Here's some basic math:
- typical 80"X50" (avg taper from 54 to 44) quilt = 4,000 sq in
- 2" loft for a 32 degree bag = 8,000 cu in
- divide by 900FP = 8.9-9oz
- add 35% overfill (3oz) to take loft to 2.6"+, good for 25 degrees
If you use something like M50, then 8,000 sq in of material (shell & liner) weighs around 5-6oz. Add 12oz of down, and you have a 17-18 ounce, 25 degree quilt.
As for your tarp, it looks like Shires' original plan. I'm a believer in making a tarp bigger rather than bringing a bivy. Again, there is less material in making the tarp larger than creating another separate item. Besides, it's a lot easier & lighter to bring window shrink wrap as a GC vs something heavier (even 1.3 sil) sewn to the bottom of the bivy.
Last, but not least, both M50 & M90 have very good DWR, which means you can cowboy camp unless it looks like it's gonna rain. With a 25-30 degree quilt/bag, you also don't need a bivy to add any incremental warm for a typical Sierra summer evening.