I really wonder who this machine is for. The primary reason to have PCIe expansion, GPU cards, isn't even a factor since MacOS on Apple Silicon doesn't support external GPUs, neither for video nor compute purposes.
Given Apple's mainstream consumer focus it really surprises me that they're releasing such a niche system. All the stuff you could conceivably do with the internal slots on this machine, you could do using Thunderbolt expansion. Is this basically a "trophy" machine? Someone with a real usecase other than "It's pretty" please chime in!
I won't mind to use it to fine-tune my drivers.
I have to look up what happened with Apple Silicon - but AFAIK, no MacOS supports (extended) message signaled interrupts, aka MSI-X on Intel. This is quite sad because MSI-X has long grey beard on Windows, Linux and everything else. One can of course write it on his/her own, but as far as IOKit (will check the DriverKit later) basics, there is no "convenient" way to implement MSI-X. Once I was about to implement that, but politics of ex-employer prevented me to do that. In retrospect I would say, I got VERY lucky: otherwise I would be unable to even talk about it.
In any case I would LOVE, LOVE, LOVE to get that MacPro and look under the hood what is about that MSI-X support.
But for a daily use I am perfect now with the "black" cylinder. A "significant part" of that black cylinder is working under Parallels in "Snow Leopard" mode. And under Snow Leopard I am working on all the legacy "9" and "X" drivers.
Hopefully soon enough I will be again on Apple Silicon, SAS and NVMe. But right now the legacy community is underserved. And in addition there is an impression about FirmTek stuff being "abandonware". I have the burden of proof that it is by no means the case.