Just playing with some numbers. The 1DX buffer is listed as 35 shots (wow) with the RAW file size of 23.2 MB. Given that:
Sandisk 160 MB/s = 53 shots minus buffer of 35 = 18 shots "written" to card beyond buffer. Times this by 23.2 = 417.6 MB. 53/12 fps = 4.41 sec. Write speed 417.6/4.41 sec = 94.6 MB/sec

Using the same methods to estimate the write speed in a 1DX for the other cards:

Lexar 1000x: 87.3 MB/sec
Sandisk 100 MB/sec: 83.5 MB/sec
Sandisk 90 MB/s: 75 MB/sec
Sandisk 60 MB/s: 61.9 MB/sec
Sandisk 45 MB/sec: 46.4 MB/sec

Then, doing the math on the Lexar 1000x in a 5DIII that I read about, 30 shots taken, 13 shot buffer, 27 MB RAW file size = 91.8 MB/sec. Very similar to the 87.3 MB/sec write speed Jonathan observed. But I am going to retest my 90 MB/sec Sandisk.