I used the i5 and an RX 590, which is a somewhat updated RX 580, with X-Plane 11. As you may know, X-Plane is very demanding of both CPUs and GPUs. I thought that the i5/590 handled X-Plane fine provided that I stayed clear of areas (e.g. Manhattan) that are highly detailed graphically, and therefore hard on X-Plane performance even on very strong computer systems.
For a number of reasons, unrelated to games, I now have an i7 and an RX Vega 56. The performance increase in X-Plane is noticeable, but I don’t think that I can pinpoint how much that has to do with the i7 and how much the Vega GPU.
On the question of CPU temperature, I am fine with the i7 temperatures that I’m getting, as reported by Intel’s Power Gadget. There are people who claim that the Intel/Apple engineers don’t know what they are doing and that the i7 CPUs will have a short lifespan. Personally, I have more faith in the engineers than in forum commenters, and in any event I have AppleCare+

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An AMD external video card plus Boot Camp works fine with the mini. There is no tinkering required. See the video in post #34 above for a demonstration. This is contrary to statements on certain Apple support pages and apparently not true of Mac laptops. However, it is clear at this point that an AMD card plus Boot Camp works with the mini on a plug and play basis.
It is not currently possible to use Nvidia video cards. However, there are some posts in the last couple of pages of the eGPU thread that I mentioned earlier about statements from Nvidia support staff to the effect that Mojave support for Nvidia cards is coming.