2014 iMac has a 2-lane slot for the PCIe bus. Theoretical max is about 1,000 MBps. (5.0 GT/s PCIe x2)
Your W-D black card would be pretty fast, but is limited by bandwidth of the slot in that 2014 iMac.
It would be much faster in the Late 2015 iMacs, which provide 4-lanes on the PCIe bus, which looks like it should allow speeds up to about 3,000 MBps. (8.0 GT/s PCIe x4)
Downgrade if you like. That won't change the hardware in your 2014 iMac -- you would still have speeds in about the same range. (which is still plenty fast!)