I use a SDXC card in my 5D MK III (its not UHS-1). I do not try to shoot the high FPS rate, so its no issue. If I were shooting high FPS, I'd merely stop recording to both cards.
If you think about it, when writing to both cards, the speed of the slowest card will limit the writing speed. Thats almost always the SD card.
Unfortunately, the design of the 5D MK III was apparently frozen before SDXC UHS-1 cards arrived. Its unfortunate, but true.