menu >about this Mac > (system Report) > PCIe slot ...
the two last lines will be:
Link Width: {x1, x4, x8, x16}
Link Speed: {2.5GT/s, 5.0GT/s, 10GT/s}
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My guess is that those will be x4 and 5.0GT/sec [PCIe 2.0], which corresponds perfectly to your tested speed of around 1500 M Bytes/sec.
This is confirmed by EveryMac.com:
Instead, the Cylinder Mac Pro models have only one internal "blade" SSD. Specifically, the SSD uses a custom Apple design with a PCIe 2.0 x4 interface, which is capable of a theoretical maximum speed of 2 GB/s.
from:
https://everymac.com/systems/apple/mac_pro/mac-pro-cylinder-faq/how-to-upgrade-mac-pro-cylinder-ssd-storage.html
You can't run a really fast drive on a much slower bus and get any better performance that what the bus supports. In 2013 when that Mac was issued, those speeds were very impressive.