Don't waste your money on a FireGL or Quadro card unless you absolutely want to blow that sort of money and absolutely need stability. I can't comment on Quadro cards, but FireGL cards are simply other r520, r480 etc cores that have different drivers written for them and have a different memory bus bitrate. x1900XTX is still significantly faster than any "workstation" card and the additional pixel operations per cycle will be useful considering CAD or graphical development is heavily dependant on vertex shader operatons, which of course the r580 excels in
So really I would look into finding some modified drivers for consumer grade cards written with the intent of more precise operations, as really that is the only difference the two cards. You'll have a significantly more powerful GPU and in any situation you can simply switch between drivers depending on whether you want the GPU to render precision instructions within CAD and less crucial operations in prewritten engines
I'm also fairly positive you want ECC compliant memory, and you don't need memory sinks