Stop cheating, John Paczkowski this time your leakers kidding you.
First the technical reality:
Thunderbolt 3 eGFX AMD XConnect specification implies disabling the internal GPU (same restriction when you have an laptop with integrated GPU as iris pro and you switch to the discrete GPU as amd radeon M380x, you cant use both at same time)
What it means for the iMac retina or the Mac Pro or the Macbook Pro with dGPU?, its respective dGPU and iGPU will be disabled while on eGPU, add this TB3 implies an bootleneck due its 4x PIE3 lines vs 16x PCIe lines available for onboard dGPU, means an eGPU has NO SENSE FOR iMac 27 Retina, Neither MBP Neither Mac Pros, so an TB3 Display with eGPU wont be a 2nd display option for its primary market, only will be an option for Mac Minis or Macbooks w/o dGPU.
Apple actually have an much better option to deliver 5K HDR @ 120hz instead TB3, Did you know USB-C also suport DP1.4 if some mac is loaded with an DP1.4 dGPU (as AMD Polaris Radeon 460/470/480) thru USB-C only ports (non TB3) it can handle even 8K display while on dGPU, NO Tricky MST, no Tricky-Expensive TB3-eGPU, only a little discrete upgrade to the USB-C only Headers.