https://www.reddit.com/r/macpro/comments/9z6epv
real world speed test results for performance minded Macintosh users
barefeats.com
There is a lot of info out there on graphics cards for MP5.1.
225W is about the limit without doing any kind of mods to power supply of MP5.1.
Even a Radeon RX590 is around 235W so if u put your system under extreme stress it may shut down or you might fry your motherboard. The RX580 is 185W so it is OK.
You want to do 4K video you are going to put a heavy load on your system.
Maybe someone else can chip in here on this thread but long term investment wise you maybe better off with those dual xeons to upgrade to Catalina and go Radeon 5000X series. The driver support is now coming out for 5700/5700XT and Radeon 5800+ cards will come to market later in 2020 I think.
Power consumption is good for these cards. No need to do any mods.
AVID Media Composer needs to be 64 bit to run on Catalina though. If you have lots of 32 bit apps you want to still use it maybe not a good thing to upgrade that far.
5700XT cards are similar performance to Vega 64 ...Radeon 5600XT cards similar performance to RX580/Vega56.