Waste of time. The limit on PCIE 2 with x4 lanes is around 1450MB/s and if that bandwidth isn't going to be consumed in the first place then buying something faster is like a 50 year old guy with a middle age complex wasting money on a Porsche. Don't fall for these 'upgrade' traps.
Doubt it'd work anyway since it's NVMe.
Looks like the AHCI flavour of the SM951 could be the last decent SSD upgrade the older Mac Pro models get.