Is coil whine common in a lot of GPUs at very high frame rates?
At very high frame rates like 1000+ yes, very common.
Some will coil whine at 60, some at 120, luck of the draw really.
Is it worth the RMA process if it whines from 240FPS and up?
No, coil whine won't affect the cards performance and is not really a defect, not a damaging one anyway, you'll only really get 240 fps in low end games/menus so you shouldn't hear it that often. You could use MSI Afterburner to limit the fps to 144 if that would be a better option for you?