So, the system is my Christmas pressy - Kaby Lake i7 based 13" HP Spectre x360 convertible (I love this notebook), which is USB-C Thunderbolt 3 eGFX enabled. The XConnect enclosure is the Power Color Devil Box which I've got wired mouse, Ethernet, a 480GB SSD and, of course, the Radeon RX 480 plugged into it. With this type of setup the single TB cable can enable all the peripherals in the enclosure, the external graphics (with cross display, in this instance) as well as power the notebook. In this pic I just had the joypad plugged in to the notebook directly to see if there were any latency differences between having it plugged in the enclosure or directly into the system.
Like I say, I'm just experimenting a little bit to see how it works. I do love the notion of the single cable enabling all that functionality and powering the notebook. Limited to effective 4x PCIe bandwidth, and sharing that with cross-display and peripheral data, is capping the GPU performance, so I'm not convinced of the need for going much beyond RX 480 level, even though the enclosure can support 300W. That image is Forza Horizon 3 at 1080p medium detail (with 4x AA) and it ran fine. There are genuine play-ability differences with Civ 6, not in terms of scrolling around the map, but in terms of how long a turn takes - the notebook is thermally limited so when you are running the entire game on integrated the NPC turns can take ages, however, use the external graphics and all the thermal budget is given to the CPU clocks making the turns come around much quicker.
Lightman - not aware of any TB eGFX / XConnect enclosures that support mGPU at this point.