That's expected, the iMac Pro is power and thermal limiting (even for the same GPU). Anyway, there is no boot screen available (for cMP, of course) for the Vega 64 yet. Unless you figure out how to flash it, or using something like Clover to initialise the GPU properly.
No boot screen and without modifications the card doesn't show up properly but for the cMP I can confirm the Geekbench4 Values. I have a cMP with a RX Vega 64 installed and it is around that value mine is even a slight bit better with a value around 190000. I hope that there will be working drivers soon in High Sierra. At the moment that is not the case even in the latest Beta.
1. If you are dropping that card into a cMP, make sure you have a plan for powering it too. AMD website is listing typical power draw as 295 watts, and 345 watts for liquid cooled. https://gaming.radeon.com/en/product/vega/radeon-rx-vega-64/
2. Don't know how crafty you are, but anything above and beyond a GTX 1080 (which has TPD of ~190W and can connect via one 8 pin) will require some extra cabling/routing or power supply mods in a cMP. 1080ti draws roughly ~250 Watts.
Yap that is true @JulianBoolean
Forgot to mention that. I also build in a second Power Supply and drive an original 5770 as well in it. Because of the missing boot screen and some other not so graphic heavy stuff...