This really depends on a lot of forces that we are unable to see or know.
But I doubt very much that AMD will release an EFI for Hawaii chips on Mac until AFTER nMP release.
As it stands, you can already put better cards in old Mac Pro. (780 & Titan) it will be embarrassing enough when tests are done and "old" Mac Pro can smoke the maxed out new one in OpenGL and OpenCl. This IS going to happen, there is no avoiding it.
But if AMD releases an EFI for 290X in next 6 months, it will be even worse. OpenCl is one place where generally inferior Tahiti can compete with GK110.
2 @ 290X in old MP would run rings around nMP in even it's home court, FCPX. I don't see them letting that happen soon.
Nvidia's moves are harder to predict. Are they peaved at being left out of nMP party? Being pushed out of rMBP entry models? Hard to say. We were able to make EFI for GK110 because it was largely already done, just required a few pieces. They either were planning cards or wanted someone to finish their work. Hard to read the tea leaves.
In short, I wouldn't count on new EFI cards, but either GPU maker could do one last set if demand warrants. But I would guess it would be well after nMP launches, too easy to steal thunder when it is using 2 year old GPUs that weren't competitive at launch. (And are further watered down with lowered clocks and clipped RAM)