Premiere Pro and After Effects does not work with AMD VEGA II

On the Mac side, it's not about the specific graphics card but whether or not the Mac supports Metal - which yours does.

At least with the current versions of Premiere Pro and Final Cut Pro, you're not going to see the same amounts of GPU usage at the same times.

My 2017 iMac Pro has the Radeon Pro Vega 56 8 GB.  Assuming I've applied enough GPU accelerated Effects to cause the Render Status Bar to turn red in the Premiere Pro Timeline, I'll see the GPU usage go up in Activity Monitor only while rendering and it's usually not more than 5.0.  If the Render Status Bar isn't colored at all (like ProRes clips in a Sequence set to ProRes Video Previews) or if it's Yellow, the GPU usually will pretty much stay very, very low (0.0 to 0.2) while editing.  Over in Final Cut Pro, the GPU kicks in while editing and while rendering, sometimes going as high as 30.0 during a render, but usually between 2.5 and 15.0 while editing.  Of course, that's based on what I'm asking Premiere Pro and FCP to do at a given time.

Even though the GPU usage isn't high on the Premiere Pro side, you should be able to observe the performace difference on your iMac Pro by doing a render action (any of the options under Sequence > Render) to a section of the Timeline that's flagged as Red by the Render Stauts Bar with Mercury enabled then again with it set to Software only.  Rendering 10 seconds of scaling a 1280x720p MP4 clip to 3840x2160 with GPU accellerated Effects like Lumetri, Noize and Basic Text applied will take about 2 minutes with Mercury enable (with about 5.0 GPU usage) and almost 3 minutes with Software only (with no GPU usage) on my iMac Pro.

If you're not already using ProRes for Smart Rendring in Premiere Pro, that should provide a very good performace boost on any Mac, especially the iMac Pro that you have.  On the After Effects side, I have found that the new Multi-frame Rendering feature nearly cuts render time in half on my iMac Pro both while working in the Comp as well as when rendering movie files.  Tha'ts currently in the public beta and should be available in the next update.

Your 16GB of display RAM should really shine when using something like Adobe Dimension or Adobe Substance Stager, but as you've noticed, not so much in Premiere Pro.

What is your Mercury Acceleration set to in the project settings dialog?

Neil

 Each app is coded differently for how and when they use what resources. So the GPU may not be working at one point, and pinged at another.

In PrPro, GPU use is determined by the effects in action at any one moment. Color corrections/Warp and resizing will get GPU use, many other things won't. They have a list of the GPU accelerated effects.

Neil