Okay I just conducted a burst test with my 1DX, using my various cards. Here's what I got:

Sandisk 160 MB/s: 53
Lexar 1000x: 51
Sandisk 100 MB/s: 50
Sandisk 90 MB/s: 48
Sandisk 60 Mb/s: 45
Sandisk 45 Mb/s: 42

(camera set at ISO 100, 1/500 sec with lens wide open, manual focus, raw, maximum burst speed).

These numbers are all very good, but really they reflect more the huge buffer size of the 1DX as compared to the card speed. The time you really notice the difference in card speed is after the buffer has been filled, and you are still shooting. Then the fast cards allow you to keep on blasting, while the slow cards hold you back dramatically. This was very handy for me when a saw a lynx family this past spring... I took 100 shots in a minute and never had to wait for the card to catch up. So if the big initial burst is all you need, then you don't have to bother with a fast card. But if you're shooting wildlife (or making videos), then the fast cards are well worth it.