I went 390 OC to 1070 and yes massiv step up. Not only big FPS boost, but less power consumption means less heat, quieter pc etc.
Vega as far as I can tell is a bit of a waste of time for gaming as it has no benefit over a 1070 (Vega 56) or 1080 (Vega 64). Run hotter, wayyy more power consumption and generally beaten in gaming by the Nvidia equals whilst being no less expensive.
I was a big amd GPU fan for years but they have lost it in my opinion.