Overclockers did a dual grahics test and if you already have an APU is a step in the right direction.
http://www.overclockers.com/testing-amd-kaveri-apu-dual-graphics-performance
they used the R7 250, which is a 384SPs card (but much higher clock than the APU, so apart from memory performance it's comparable), while the r7 250x is a 640SPs card also with much higher clocks, and it's also an older GPU, r7 250x = 7770.
I don't even know if they enabled driver support for "dual graphics" with anything other than the 240 and 250 (non X) for Kaveri, and if they did I think it's not a good solution, the difference between the kaveri IGP and 250x(7770) is very significant, and probably the gain over the 7770 alone is going to be small, and CF can add problems to the experience, my guess is that it's probably not worth it, just go with a faster card with no support for dual graphics, like the 260x the price difference is normally small anyway