Very.
Also you can sell your RX 480 to make up some of the money. They still go for good $$$.
I'd say it's probably worth it performance wise even on a 1080p monitor, but a 1070 ti might do the job better. Looks like the cheapest 1070 ti on newegg right now is $529 and the cheapest 1080 is $630.
Personally I wouldn't pay those prices unless I absolutely had to have more performance now. If that's you then go nuts
I'd save the $100 and go with the 1070 ti though. You won't miss those 3 frames at 1080P and you should stay above 60 FPS for at least a few years.