I ended up cancelling my order for an iMac with a 580X and re-ordering with a Vega 48. After a lot of research, I ran across a couple major issues with my original plan, which was to use the 580X for a few years, and then upgrade to an eGPU if necessary.
1) eGPUs take a big performance hit in some apps and games because the max bandwidth of Thunderbolt 3 is 4x slower than the PCIe connection to an internal GPU, and it becomes a bottleneck. In many apps and games, the internal Vega 48 performs nearly on par with a Vega 64 eGPU, and in some instances, the internal Vega 48 will even beat Vega 64 and Radeon VII eGPUs. In cases where the internal Vega 48 beats both the Vega 64 and Radeon VII eGPUs, the Radeon VII doesn't even perform any better than the Vega 64 even though it's a faster card, because once the Thunderbolt 3 connection becomes the bottleneck, it doesn't matter how fast the eGPU is. You can connect an eGPU that's 4x as powerful as the Vega 48 in 5 years, but if the app or game needs to push more data to it than Thunderbolt 3 can handle, it still might perform worse than the internal Vega 48 in the real world.
In my opinion, eGPUs are a great solution for improving the graphics performance of laptops with horrible internal GPUs, but once you get up to a decent midrange card like the Vega 48, an eGPU isn't really a viable upgrade unless you are using one of the few apps that support using multiple different GPUs / eGPUs at the same time. No games can do that, so for gaming, I think the Vega 48 is as good as it's going to get for the current iMacs. Now, if you're really serious about gaming, maybe you just buy / build a dedicated Windows PC for that, but as for me, my iMac is mostly for work, but I want to be able to do some casual gaming on the side, and I don't want to buy, maintain, and clutter my desk with an additional computer just for gaming. The whole reason I want an iMac is for the clean, minimal design and the beautiful screen, both of which you would throw out the window if you have to put another computer and monitor on your desk.
2) eGPUs only work okay with an external monitor. Like I said above, Thunderbolt 3 seriously bottlenecks eGPUs, and that's when using an external monitor. If you want to use the beautiful screen that came with your iMac, the Thunderbolt 3 connection becomes even more of a bottleneck because now all that data has to come back to the iMac over the same interface. You will never match the performance of an internal Vega 48 doing this. Thunderbolt 3 just doesn't have enough bandwidth for this, and most apps and games aren't compatible anyways.
What really needs to happen to make eGPUs a worthy upgrade for mid-high end internal GPUs is adding more bandwidth. Maybe that means connecting multiple Thunderbolt 3 cables to eGPUs, or maybe Thunderbolt 4 will solve this issue, but for now, I would seriously consider paying the extra $300-$400 for the Vega 48 if you want the best possible graphics performance for your iMac. Spending $600-$1200 on an eGPU enclosure and a Vega 64 or Radeon VII just to get barely better, or sometimes worse, performance than an internal Vega 48, plus losing the ability to use your beautiful 5K screen, just doesn't make sense to me.
