I'd like to share my experience.
I bought this Mac a week ago:
MacBook Pro (15-inch, 2018)
2,6 GHz Intel Core i7
16 GB RAM
512 SSD
Radeon 560X
It's the top tier model without modifications
Also I bought the Razer Core X eGPU (320€) and I already had at home a MSI Gaming X 1080 ti 11G and a Asus PG279Q 165Hz external monitor.
Playing without the enclosure is a matter of lowering the resolution a lot, but you can play at recent games at 30-40-50 fps. I prefer to keep the resolution lower (think 720p) and graphics quality on high.
Playing with the Razer Core X and 1080 ti is almost a dream come true. It's the closest thing to plug and play I could hope for: I just have to connect the eGPU when the Mac is completely off and then just boot in windows (no tricks at the startup screen required). Stop. I'm talking about external display use obviously. No hot unplug allowed or it's a blue screen. First turn off the Mac, then unplug the eGPU. I didn't even try honestly using the eGPU on the Mac screen itself since I prefer gaming on high refresh display.
The first time I connected the eGPU I had to manually install Nvidia drivers and that's all.
With this setup I can use ultra settings (everything maxed out at 2560x1440) on every demanding game I can test, including Witcher 3 and Rise of the Tomb Raider and still get more than 60fps, think about 60-100 depending on the scene.
If I play less demanding games like Overwatch (everything maxed out at 2560x1440) the fps are 120-165 (max monitor refresh rate) depending on the scene, perfect for competitive play.
Notes:
- If I take the graphical options down a little bit I can get solid 165 fps easily, and 100-120 on the more demanding games, always 2K.
- I'm also happy to use the Mac keyboard, I was coming from a Razer Blackwidow Chroma V2. External mouse is a must for competitive games (I have a Razer Deathadder Elite).
- Long story short I sold my PC to a friend, now I'm living the one machine dream after so many years waiting

Only one cable and the mouse

- I didn't test eGPU gaming under macOS since I know there is no support for Nvidia eGPUs and I don't want to mess with my main macOS partition since I work with it
Problems still to solve:
- The only tinkering I had to do was install and use Intel XTU to calm down my Mac under Bootcamp while gaming, think about 30W max use, it depends on the temperature of the room, since if I don't do that the system will throttle every 5 seconds after a minute of gaming and make every demanding game TOTALLY unplayable. I hope Apple releases a Bootcamp update that resolves this issue. This is the only nuisance, it's incredible Apple didn't test this problem before shipping, unacceptable.
- In Rise of the Tomb Raider I have problems with sound if I play with the Mac speakers, crackling sound and various noises, I understand this is a common driver issue, I heard that maybe Mojave beta doesn't have these issues. No probs with headphones