I expect 10-20% better performance tops. nVidia does not need to make something 100% better than their older cards.
With close to no competition, they can every year or two release something little bit better than the previous product.
Why would they compete against their own card?

I really don't see the point make something twice better than your previous product when you have the best tech and nobody is close to you.
AMD needs to step up his game. Vega was disappointment.