At least for v3 Intel Xeon processors here is what I had to do to get Dell Precision T5810 T7810 or T7910 workstations to boot off of NVMe flash drive on a PCIe card – real simple.
Update the BIOS to the latest BIOS update – that will allow the legacy boot, or UEFI boot manager to see the NVMe drive that is on the PCIe card. On my T5810 as of 02/17/2017 that is A16 version – get it from Dell’s support website with your Dell Tag#.
BTW – make sure you put the card in a gen 3 X4 slot or higher (X16 is good too) – that would be slot 0 X4 – or one of the two graphics card slots – X16.
Hope this helps… Mike