The heat sink on a MPX GPU has thicker fins than normal, with a wider than normal spacing of said fins as well; this is needed for proper airflow & heat removal since the only air is coming from the front fans... Now, if the power draw of an Apple GPU is significantly less than that of 'normal' mid to high end GPUs (250 to 400 watts these days), they might be able to get by with a 2 or 3 slot heat sink, rather than the four slot monster that is the current norm...
Just because they are switching to their own Apple silicon APU / CPU / GPU / whatever does not mean they are suddenly going to shrink the goodly-sized single-slot Afterburner card into a sliver of a speck of silicon in aforementioned computing chip...
Now, being that the main compute chip n a reduced-size Mac Pro is supposed to include a GPU, an add-in GPU is not required for functionality, meaning the MPX slot is open, as are the ones it would normally block...
So audio dudes could have four full-length PCIe slots for their audio add-in cards, and a half-length slot up top for the I/O...
Video editors could add a GPU in if they want, but they would still have the slot open for an Afterburner card... Video dudes may prefer some sort of A/V I/O card as well, or a few M.2 NVMe SSD RAID PCIe cards...?
Seems like the (possible / potential / whatever) reduced power draw from the transition to Apple silicon will make for reduced cooling needs, making for reduced mass needed in semi-passive heat sinks... Smaller chassis...!
But come on Apple, put that same reduced package Mac Pro mobo into a less 'fussy' chassis, give the masses a lower cost Mac tower, give us the mythical xMac...!!!
EDIT TO ADD:
I would hope, reduced-size Mac Pro or xMac, for four full-length PCIe slots (bottom one a MPX slot?) & one half-length PCIe slot (for the I/O card)...
Then I would want an Apple GPU (2-slot, but still the semi-passive heat sink), a single-slot M.2 NVMe Gen4 SSD RAID card, a single-slot Afterburner card, & the single-slot half length I/O card could all fit in there...!
Use the a few of the TB4 ports for A/V I/O & a control surface or two, a few more for a couple of monitors & you would have a complete personal Digital Content Creation workstation...!
The modern day SGI O2...!
I would just rather forgo the fancy post modern cheesegrater grill & space frame chassis (with useless handles & feet or wheels) for a cost reducing (yet still a quality Apple design) chassis; and not needing a 1.4kW PSU has to save a few bucks as well...!
Come on Apple, xMac...!!!