Sorry, but I don't understand all the talk of motherboard - soldered or not - RAM on conceptual Mac Pros. M architecture has shared memory with the GPU, using wide data paths.
It seemed to me when Apple introduced the Max processor, its difference to the Pro was data path and GPU transistors. The earlier M processor has 18 billion transistors; the Pro has 33.7 billion. The Max has 57 billion.
And these processors share the memory between them - and only up to 64 GB on the Max.
So IMO, the logical outcome for a desktop, is if you want to increase performance, you add in more of the Max processors. Who knows if Apple would build a Super Max which had 90 million transistors, it's increase being for GPU processing? They'd only be selling that to desktops, and most likely, as GPUs. It would not make much sense unless the same processor found its way into a notebook product.
It might be more sensible to design a motherboard with very wide data paths and allow each CPU to add the other one's shared ram. I think I've seen this approach with the Borg on Startrek ...
I reckon the retail price on the Max processor is $400 each. That is cheaper than many workstation GPUs, isn't it?
So if you bought a Mac Pro, I guess the question is, how many CPU's might it support? If it supported 8, then that would cost a customer 8 x 400 = $3,200 (for 8 Max processors). Who would also get 8 x 32 GB RAM = a quarter of a terabyte of RAM.
If you up the RAM, then Apple charges $400 for the extra 32 GB, per processor which relatively expensive.
For their current Mac Pro, Apple charges for two Radeon Pro (the proper pro vector suitable reliable etc) GPUs, with 32 GB each, erh hhh $11,600. The 9600 does have 60 billion transistors. Slightly more than a single Max processor. Compared to buying 8 Max processors, at my $400 each. For $3,200.
Apple charges $3,000 for 384 GB RAM on the Mac Pro. OK that's expensive compared to buying it yourself. But if one bought 8 Max processors, you'd get 256 GB of RAM included with them, for $3,200. But to get an extra 32 GB RAM with each processor, Apple charge $400 extra per processor on the Macbook Pros, so it would cost you $3,200 to get half a terrabyte of RAM, or double the 256 GB RAM. So you pay a fare bit for the extra RAM. Big deal I suspect if this model came into existence. But:
Surely the way to increase GPU performance on an M Pro Mac is via expanding CPU/GPU processors? They'd be a cheap way to do it if the motherboard could handle multiple processors.