Overall, the Radeon Pro W6900X is somewhere between an RX 6800 XT and an RX 6900 XT. Since it is noticeably faster than the RX 6900 XT in the Metal API tests, the professional card could be getting 32 GB of VRAM. The clocks will most likely be lower though, so it might only feature a more compact blower style cooler.
https://www.notebookcheck.net/Apple-Mac-Pro-exclusive-AMD-Radeon-Pro-W6900X-GPU-pops-up-on-Geekbench-beats-RX-6900-XT-in-Metal-API-tests.534553.0.html