I dunno I think this varies based on what you're playing. For example if I'm playing an FPS with no HDR support(OW, f ex), I might turn BFI on and play at 1080p integer upscaled to 4K if I need to peg 120fps. That should be a better experience than anything short of a CRT. And some FPSes(CSGO probably?) can easily peg 120fps even at 4K. Hell, maybe you can peg OW at 120fps with a 2080TI or upcoming 3080TI -- it's certainly not difficult to drive.99% chance Gsync and BFI will not function simultaneously so it's completely worthless for me.
Of course, if you're playing graphically intensive, new AAA games, you aren't easily pegging 120fps at 4K, but in that case you may care about HDR, and it probably won't be an FPS, so who cares about BFI then. I mean, sure, BFI is a nice to have for something like Cyberpunk 2077, but given a choice between BFI and HDR I'll take HDR 100% of the time and twice on Sundays. And it's unlikely BFI will be compatible with HDR for a very long time if ever.