I use a Radeon RX 580 in a Razer Core X with my 15" MacBook Pro 2017 daily. It's great. I highly suggest going for something like this over the black magic and other premade solutions, since you will want to upgrade your GPU someday, and buying a new enclosure every time doesn't make sense.
I haven't ran benchmarks to see if there would be a dramatic difference between internal/external performance, but I can say it runs very well as I expect.
The eGPU keeps the laptop much cooler. I do fairly heavy 3d game development. Using the internal GPU, my laptop gets to 70 degrees or so. Using the eGPU, it sits around 45 degrees instead.
All in all I strongly recommend eGPUs for Mac users that need graphics power. It solves two of the biggest Mac hardware problems; heat and hardware options.
Honestly, I often wish I got a 13" MBP instead of the 15" - I didn't expect I'd use an eGPU this often, and the 15" seems to have more problems.