The issue with SD card speeds is that the 5D MK III does not support UHS-1, and most new cards base their speed ratings on UHS-1, which has a bus that it 2X faster. Cut those speeds in half.
The big issue that many are fooled by is that SD card speeds are based on a new totally blank card. As long as the cells in the card are blank, the camera does not have to do the slow block erase function before it can write to them.
In the real world, we use our SD cards, and depending on the size, they get filled with data sooner or later. This coupled with the belief that formatting in camera somehow blanks the card seems to be missed. There are two types of in camera formatting for SD cards, regular formatting which just updates the fat tables to show that files are available to write over (SLOW). The second type, called low level format, actually erases the card and returns it to full speed. I wonder how many use this function more than once. Its very slow to do a low level format of a large card.
So, with a used SD card that has been formatted in camera, the 5D MK III will achieve somewhere between 5 and 10 MB/sec, no matter what that label says, be it 60MB/sec or 100, its going to be 10MB/sec or less.
Once Canon starts supporting UHS-1, the speeds will double, but... that means 10-20MB/sec once you are in the mode of writing over used memory cells.
CF cards don't have that issue.