Samsung just released a firmware update that boosts bit rate up to 80Mbits on 1080p modes among other improvements. Time to test.
UHD downsampled to 1080p shows a clear improvement in image quality vs just recording at 1080p. So yep, it's the way to go unless you need slow motion.
Testing Gamma modes
Does the DR gamma mode get extra dynamic range? Yep, but only on the shadows. Highlights remain the same.