Errr, no. PCIe v3 x1 bus limits are 8 Gb/s ( of about 984 MB/s ).
https://en.wikipedia.org/wiki/List_of_interface_bit_rates
The Mac Pro SSD is hooked to PCH so it is PCIe v2 x1 which is worse at 5 Gb/s (500MB/s )
Those x4 NVMe drives blow right past that in bandwidth. Those are not on a x1 connection. Even the original MP 2013 drive blow right past that cap. It can't be a x1 link. PCIe v2 x4 is 20Gb/s ( ~2GB/s ) which is enough to do the original and the newer NVMe versions.
It is a x4 link. The 'cap' is more so the SSD controllers and NAND configuration and properties. Plus power and thermal limits. NVMe has some lower latencies and a few other updates to squeeze a bit more out of the link but still v2 x4 is plenty.
The notion that the Mac Pro SSD is capped or throttled doesn't hold water. Apple itself later shipped faster SSDs on the same base infrastructure.
Thread circa 2017
https://forums.macrumors.com/threads/new-macpro2013-disk-speed-confirmed.2043315/
And again all hooked to the PCH and PCIe v2 x4. a x1 link would hobble them down near SATA drive speeds; there is not much point to that by 2013 tech levels. You probably could get limited performance out of the small capacity drives, at that but that wasn't the base connectivity link. (that is more so just low parallelism to the NAND chips. )
At some point in time, a few laptop instances may have been pragmatically capped at PCI-e v2 x2 but x1 speeds is far more likely to be some SATA derivative than a based PCI-e link.
Apple's blade form factor isn't standard, so off the shelf Intel SSDs aren't an internal option.
P.S. there is a block diagram for the Mac Pro 2013 (6,1) that has been around for ages.
https://forums.macrumors.com/threads/proper-manual-for-the-new-mac-pro.1768967/#post-19531548
How the SSD is connected is clearly marked there.