I also cut the sleeves at size 4, moving the notches an inch to accommodate my changes to the armhole. I basted it at 5/8 inch wrong sides together to check fit and work with the ridiculous amount of ease in this sleeve. Then I stitched it at a quarter inch for a French seam. (This stuff frayed a lot, which made that quarter inch tricky.)
With all that ease, I ended up with a few unintended gathers at the shoulder. But it's barely visible in the red and gold pattern, so I decided F it. They stay.
It is a tight armhole. Bad posture is impossible.