While helping someone who was having less than expected performance from their SN770, I noticed that it had the same firmware bug as the SN750 as well as the SN850. While the payload size issue was fixed on the SN850 via a firmware update
The issue stems from a firmware bug where if the SSD is connected to a M.2 slot that runs through an AMD chipset, then it will use the lowest payload size rather than the payload size supported by the SSD.
Running firmware 731100WD
These issues only happen when using the m.2 slots connected to the AMD chipset, but not the first m.2 slot which has a direct connection to the CPU.