I think Apple says 11 TFLOPS Single Precision and 22 TFLOPS Half-Precision, which puts the GPU at a Vega 56 I think? I don't think Apple has said how many possible GPU configurations there are, so you could be able to bump it to a 64.
You're correct -- the specs page of the new iMac Pro is intentionally vague, but they do confirm this much at least: https://www.apple.com/imac-pro/specs/
(The 64-variant will actually have 16GB HBM2, instead of 8, though.)
They're certainly not putting a 300 Watt gpu in an all-in-one enclosure. The only way that would work is if they used the closed loop cooler version, which is too bulky.