There's no perfect pairing, every games requirements is different. If a cpu is capable of 100fps in one specific game, all that's required to max out details is a gpu strong enough. Going stronger does nothing, you won't raise the fps, that's set by the cpu, you'll just have an excess of gpu power. If the gpu isn't strong enough, can only do 80fps, you lose that 20fps as the gpu can't sustain the fps the cpu gives it. However, that's at that game, you could always play a different game, where the cpu is maxing 60fps, in which case that weaker gpu is now plenty strong to max fps.
So it's a constant balancing act between cpu fps output and gpu ability to reproduce the fps. And it's totally game dependant as to which way the balance tips. With a card like a 2080/ti, the balance is heavily weighted in gpu favor to start with, so choice of game is mostly moot, there's only 1 game I can think of that'll hurt even that card at 1080p. But for 99.9% of all other games, that gpu will still be half asleep and still max out details, seriously over powered for 1080p.
The gtx1660ti has replaced the 1070 in nvidia lineup as top end 1080p card, the 1070 is pretty much no longer produced. With a price difference of $280 vs $420, it's a really bad buy now.