Has anyone done any r/w performance comparisons using btrfs with compression enabled on an M.2 nvme ssd vs uncompressed hdd?
It looks like Phoronix has done some benchmarks of BTRFS and compressed BTRFS on both SSDs and HDDs that might give you a general idea of performance compared.
Of course, your real results will depend on both the options you set (which compression algorithm you have BTRFS use, and at what level of compression) and your hardware's performance (both disk speed and how quickly compression may be applied), plus other variables (other applications running, what software/firmware is used, etc).
You may want to start by doing small benchmarks of your own using your hardware. Then you can record your results and compare them to find what will work best for your scenario.