I normally use Sandisk Extreme Pro CF cards in my 5D M3 and then several people told me that they don't fill the buffer using a 1000x CF card. I tried the Lexar 1000x card and the buffer still fills up. I seem to get a few more shots before the buffer fills with the Lexar 1000x.
Has anyone tried other cards that are fast enough so that they buffer never fills?
You want to avoid RAW+JPG though as the buffer for that can still fill fast. JPGs go without filling up. RAWs go for a long time. RAW+JPG can hit the buffer pretty easily.
I do highest quality raw + lowest quality Jpg... so not best of both worlds... but good enough.