Using eGPU to improve graphic performance on iMac 5K 2017

Apple offers the Blackmagic eGPU equipped with a Radeon 580 Pro, which is the same card already installed in my iMac. So my first question is: does this eGPU improve the graphic performance of my iMac if it uses the same card? Secondly, if this is not the case, then which graphic card should I consider to get the highest performance boost? On their support page they suggest several ones which can be used with the (rather cheap) Razor Core X housing. BTW I’m still on MacOS Mojave 10.14.6. and don’t plan to upgrade in the near future, so the cards mentioned at the end of the support page don’t qualify. Any input is most welcome!