I think the big thing that keeps me waiting is that the W5700X is likely going to be a lot cheaper than the Vega II. The big question mark really is what sort of premium does the MPX version carry over the W5700.
It really depends on how hard your process pushes the GPU. What I'd do is actually have Activity Monitor open and run a GPU history window while doing your workflows where you are looking get extra performance. If your GPU history is maxing out during those, you can probably get more performance from an upgrade. If it's not then you probably won't get a huge boost, and it's probably better to save your money.