You should refactor for a Xeon, not an i7 Extreme. Though there's no selectable tech specs in the Apple Store for the iMac Pro models, there is an 8-core Xeon presently available that retails for $2900. Account for an Intel price drop on this CPU to split your i7 estimate against mine and we're looking at $2000, not $1080.
As for the AMD Vega Frontier price point, we'll have to wait until SIGGRAPH 2017 for the details. End of next month.
Svartalfiar fails to grasp that there is a vast difference between a consumer grade computer that he can build out of the parts that he has priced out (and as he claims he can find even cheaper versions to include to get the price down and also lower performance) and a professional grade workstation computer, one which are seldom built from the ground up but are most often bought with support contracts from major computer manufacturing companies, which are willing to stand behind their reliability.
Tim Cook stated in the Keynote presentation that the iMac Pro is utilizing the latest Xeon, one that is not yet available for any other manufacturer but Apple. I am pretty sure, however, that by the time the iMac Pro is shiping, the new Xeon will be available for everyone else as well. Frankly, I wish Apple would have these available to ship the day of the announcement, or at most a week after they make the announcement. . . not five months later. That is either poor planning on Cook's part or I suspect it has something to do with regulatory environment and the FCC licensing due to all the new product hoops required by the government. There would be no way to keep it secret for those months anyway.