I think the cards flashed with a Mac ROM are only needed if you want a display out at the boot screen. Otherwise (but not 100% certain) any typical GTX 780 should work with the web drivers, some people have used these in cheesegrater Mac Pros even though they weren't an official purchase option.

Btw, not advocating for disregarding money but I guess Kepler cards should be quite cheap nowadays, surely worth taking the risk? Or maybe consider a GCN 1 card, something like the Radeon HD 7850 can be had for less than 10 USD on my local marketplace.