You can't also get more than ~1450MB/s for two other reasons, the Intel DSL5520 Thunderbolt 2 Controllers are still based on PCIe v2.0 tech (x4 connection) and Apple connected the three Intel TB2 controllers to a PLX PEX 8723 switch configured to 4 PCIe v2.0 x4 downstream ports:
You surely can get better performance than the internal SSD (very easy if you got an older MacPro6,1 with the original SSUAX, the first SSD model used with all earlier than 2018-ish built late-2013 Mac Pros, the later made ones had the improved SSUBX and some people say that the very last made even had the SSPOLARIS), but not a better throughput than the double bottleneck of the x4 PCIe v2.0 connection.
