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I recently went from a GTX970 to a Vega 56 and so far it's been spot on for me (specs in my sig.)
I suppose the answer for you depends on the rest of your system as to whether you are going to create a bottleneck and the resolution/refresh rate you are going to game at!
I would have probably gone for a Vega 64 but didn't want to change my PSU......and then ended up changing pretty much every other component, including the PSU, for my Ryzen setup!