I don't use a 1DX, but I use Sandisk Xtreme 60MB/sec CF cards in my D800.. which has to shift 50MB RAWs around, and it does OK. The 90MB/sec cards have never been necessary for me, but I rarely shoot images in rapid succession. If you're regularly filling the buffer when shooting RAW then the fastest cards you can should be what you're looking for .
There are some useful test figures here... he's not updated the camera database for a while though... so no 1DX. Still a useful like for like comparison of CF cards.
BTW... that a SN95 Mustang in your avatar pic?