One thing to note here that's a bit ambiguous in your comment - an M1XX with 128 GPU cores would still have substantially fewer GPU cores than the 2x Radeon Vega II Duo.

The Radeon Vega II Due has 128 compute cores and 8192 stream processors, so that maxed out mac pro would have 256 compute cores and 16384 stream processors. I don't have any benchmarks to work off of, but I think it's safe to say that a new Mac Pro with the 32-core SoC would still be substantially far behind a current Mac Pro unless it also had a dedicated GPU.