Yes the 1DX can take full advantage of UDMA7, but so can the 5D3 and even my old 5D2 could use it after a firmware update. I'm a big fan of fast cards, because nothing drives me crazier than having to wait for a buffer to empty when there's a rare animal like a wolf right in front of me. I have used the 100 MB/s card in my 1DX and it is very, very good. I also just picked up a 160 MB/s card and notice a slight improvement (I haven't actually tried counting the burst yet). The speed rating refers to the fastest it will ever do, and my understanding is that real-world performance is always a bit less. So I don't think we can expect a 100 MB/s card to sustain 5 fps with 20 MB files for any significant length of time.