I don't see the point in plucking down big bucks for a BTO iMac with an upgraded video card, at least not anymore. Before, it would help you to get a marginally better card that was still way behind the rest of the industry performance-wise. Personally, I'd invest that money into an eGPU now that they're going to be supported, assuming they fix the problems with internal display acceleration.
I think that in the not too distant future, you're going to see Macs in general rely more and more on the I/O afforded by Thunderbolt 3. The machines will stay slim and sexy, but you can stash a bunch of things under a desk or in a cabinet to do the heavy lifting. After all, Thunderbolt 3 basically provides the same sort of PCIe connectivity that motherboards on high-end gaming PCs do today for cards installed in the PC itself. For Macs, those cards will just need to be connected via wires and eternal enclosures.
That's what the MacBook Pro and now iMac Pro's business model is today, and as Thunderbolt 3 peripheral prices go down (Intel is trying hard to waive the royalties and drive up adoption), you're going to see more and more non-pro products hit the market. As costs go down, I can foresee spending $300 total for an enclosure and eGPU that totally kills whatever the iMac shipped with in about a year or two.