Hey Angus! 
I use a 2080Ti OC and im gettint good performance. Id say generally about 40-50FPS in PMDG at bigger airports on Ultra.
Im about to upgrade my CPU very soon and id like to see it it will improve further or if i really need a new GPU soon.
Right now, i think CPU is more important.
However, i lock my frames with vsync to 30FPS, so i have a smoot experience woth ultra graphics settings. If you want 60FPS stable, you might need a 4090 with frame generation.