Thank you.
My goal is to keep the broadcast/stream at 854x480, but I switch between retina "looks like 1024x640" (really 2048x1280), and actual "1280x800". Of course, doing this means I need to change the original viewport size.
Things are much faster if I disable the retina double pixels, but the software I have that does this won't do it at 1024x640, and 1280x800 is much tinier to my eyes.
(And a side effect: text changes from crisp to blurry in existing windows unless I reload what's in them.)