I have a couple Transcend Class 10 UHS-1
, which are capable of 10 MB/s write speed. They work great in the X-Pro1. No problems.
Haven't measured them against a slower card in camera, but I would assume faster cards are faster.
I prefer these UHS-1 cards since I use a USB 3.0 card reader. I see read speeds of 130-150 MB/s with UHS-1 cards on a USB 3.0 port. The older SD cards tend to top off at about 30, 50 MB/s due to the limitations of the card itself.
Personally I don't machine gun much, so I haven't encountered any 'buffering' wait times with the X-Pro1—or the M9 for that matter.