I’ve been able to get a pretty good experience using an old mac pro tower (before the round design mac pro) and replacing the graphics card. nvidia have a webdriver available that enabled me to use a GTX970, and a few weeks ago they released a driver that supports 10 series cards so I have upgraded to a 1070.
Metal has really helped get closer to feature parity on the graphics API front, which for years was held back by macOS using a version of openGL that did not have parity with DX11-level stuff on Windows. It is a shame that Apple dropped the ball with pro-end desktop hardware but at least they admitted their mistake and tried to reassure users that a mac pro that will be properly upgradeable is on their product roadmap.