I am itching to upgrade to one of the top of line GPUs too. So with that said I was holding out on the release Radeon VII, and then hearing about Navi I held off for that. So yes there are much better options than the RX580 when considering solely performance. I keep holding off my upgrade due to newer gear seems to keep changing my mind. There are some caveats of all four GPUs you listed though
Since you are still on Sierra I am not certain any of those GPUs will function, I am under the impression there are no drivers for any of those GPUs in Sierra. To run Vega 56 or 64 I think you want to be on High Sierra 10.13.4+
Radeon VII I think you will want to be on Mojave 10.14.5+
RX 5700 and RX 5700 XT were literally released this week so there are no drivers for macOS as of right now. Only time will tell if drivers will be introduced for Mojave or if it ends up being exclusive Catalina and future macOS releases
I believe Vega 56 can be powered by the internal mini 6 pins. Vega 64 and Radeon VII though use like 295 or 300 watts so you cannot get that from the mini 6 pins. You need to either use power from unused SATA ports, an external/auxiliary PSU, or mod the cMP's internal PSU with the Pixlas mod to power those GPUs properly.
I also believe there are fan behavior issues on the Vega 56/64 GPUs which was not corrected until a recent Mojave update, which again is 10.14.5
do you want the best GPU for gaming and/or workstation type tasks? Initial reports are suggesting the RX 5700 XT is pretty comparable to the Radeon VII for games, but if you have workstation type needs (such as video editing) then the 5700's sound like could be performing bad in that area. So that could be critical based on what you want to use the GPU for.