My own personal port report: Graphics are very, VERY nice and it runs super well on my PC. It supports ultrawide once racing, and I seem to hover between 95-100fps with a gtx 1080 OC'd. HOWEVER, there was one moment in the race where it dropped to 3fps or so for about 3 seconds, before getting back to normal. I also noticed that every 4 to 5 seconds, the framerate counter on my monitor would very rapidly show about 75 or 80, then immediately get back up to 95-100fps. I couldn't feel it with gsync as such but if my performance was lower, I'm certain I'd notice the slight hitching.
It also doesn't help that it just doesn't 'feel' as fun as FH3, so I've decided it isn't for me. Uninstalled the demo, though SUPER grateful that they supplied one at all, it's a great move. Hope forza fans find a lot of enjoyment in the full release!