The 290x is a hot GPU requiring high-end custom cooling just to maintain its advertised clock speeds. An "enhanced" 290x, i.e. 390x, with higher clock speeds is only going to eat into that thermal overhead even more, requiring more cooling and offering even less overclocking ability. And then, it will still be dependent upon AMD driver support, and it still won't do PhysX, DirectX 12.1, Adaptive VSync, HBAO+, MFAA, FXAA, etc.
The 390x is a last gen GPU being produced by a company that is too strapped for cash to actually put out a true new generation as a series of cards, rather than just one card. Even if it did beat a stock GTX 980 by a few insignificant FPS, which it likely won't, do you really want to climb on board that...