Blackmagic Forum

Has BM improved AMD GPU performance"
https://nofilmschool.com/2018/07/blackmagic-gpu-expander
"Blackmagic engineers have worked hard to ensure that AMD GPUs are well supported by its popular color-grading software. At this point, NVIDIA and AMD are performing roughly equally with Resolve. Blackmagic's choice to integrate a native AMD chipset is a big endorsement from the company, signaling that processing will continue to be competitive with NVIDIA and that it will keep Resolve working well with AMD in the future. "

I thought the consensus was Nvidia performance was better than AMD and that a Win/Nvidia system was better than an Mac/AMD combo according to the Forum and tests from Puget Systems https://www.pugetsystems.com/labs/articles/DaVinci-Resolve-14-iMac-Pro-Mac-Pro-vs-PC-Workstation-1154/ .
"DaVinci Resolve is one of those applications that is simply hands-down better on a PC. A big part of this is due to the fact that Apple uses AMD GPUs in their systems and consequently is not able to utilize the CUDA framework that NVIDIA cards offer."

Is a T3 external GPU as fast as internal bus?

My GPU knowledge is limited to the the recommendations so all comments appreciated. Thanks

For a long time it was consensus that Resolve is running better with Nvidia, but it's true that they are supporting AMD now as efficient. You can build a very fast system based on Ryzen and AMD GPUs, check the threads here in the forum.
That said, Apple is constantly improving their Metal technology and that eGPU will make for a nice, above middle ground system with an iMac with the same GPU. But even TB3 is at it's limit when it comes to GPU traffic, internal GPUs are the better solution.