Is the RX 5700XT more powerful than the Radeon RX 580 8GB non-OC ?If you don't need backwards compatibility with Mojave, a RX 5700XT can also be used with Catalina/BigSur/Monterey.
You will need an eVGA PowerLink to balance the two power rails, btw.
eVGA PowerLink is far far too complicated for me and dangerous because I'm not an electrician but a plug-and-play guy