I've read a blogpost from Jeff Cable Photography, a fellow photographer that uses a Canon EOS 5D Mark III, who found out a slight issue with the 5D Mark III SD slot which gives a slow buffering when using both the CF slot and SD card both altogether. But i think this will not be an issue for those who don't really care for memory card speed. It is said that the 5D Mark III has a max of 133X speed of SD card so meaning that even if you have a 600x speed SD Card or 1000X it will still buffer at the speed of 133x. The only speed that you will get is only when you are transferring the pictures into your pc, or macbook.
Considering that it is a limitation of the hardware, this problem can't be solved even if there is a firmware upgrade which is quite a bummer for Canonians that uses 5D Mark III. But hey it's not really a problem right because you still have the CF Slot. Even the Lexar Card that supports UHS which stands for Ultra High Speed where the Lexar has 1000x speed is still very fast and you can take advantage of this.
Here's from Jeff himself,
Well…after some testing I have determined that, if you care at all about high speed shooting or clearing you buffer quickly, YOU DO NOT want to put a card in the SD slot. Why? Because, for some reason unbeknownst to me, Canon decided to build the 5D Mark III with one very fast CF slot which supports the newer UDMA7 protocol and a standard SD card slot which does NOT support the high speed standard (called UHS – for Ultra High Speed).