Definitely. It should deliver at a minimum 12.5TFLOPS of single precision compute performance. The Vega RX 64 delivers that and the Vega Pro 64 will be at least that fast, but probably faster. The GTX 1080 delivers 8.9TFLOPS. The 6.1 Mac Pro delivers 0.8TFLOP/s per GPU, for a combined total of at most 1.6TFLOP/s, but usually programs could only take advantage of one GPU, and even then probably only at 80% efficiency.
As a result, the iMac GPU solution should be around 8-10 times faster than the trash can, in most cases it will be almost 20 times faster, and by the way, the trash can is slower than the current gen MBP.
I've been wailing like a banshee for proper GPU's in Mac products for years now. Looks like I might finally get my wish! If the new modular Mac Pro has anything even remotely like this, I'm buyin' it!
The Vega Pro 64 in there runs at 83% of a desktop ones Gflops (and working backwards from that – clock speed) and 83% of the memory bandwidth, for thermal considerations for sure. 11Tflops, 400GB/s. Not sure where you got the impression the Pro variants would be faster, they're always clocked more conservatively for reliability and heat.
https://www.techpowerup.com/gpudb/2871/radeon-rx-vega-64
https://www.apple.com/ca/imac-pro/
And your minimum is the maxed Vega Pro 64, the Vega Pro 56 is something lower.
The Mac Pros D700 was also 3.5Tflops * 2, did you take the worst one to compare to?
https://www.techpowerup.com/gpudb/2555/firepro-d700