Had some spare time, found a stop watch and a handfull of cards and did some quick testing. This is time to finish writing 15 shots at 4FPS, FX mode.
There are two interesting things about this test, and would be curious if anyone else can confirm.
The first is that performance seemed to stop improving when the CF card hit about 60MB/s. The Lexar 1000x card showed no significant difference vs. the Lexar 400x. That's big news if proven out since the 1000x is a LOT more expensive.
The other is that using the cards in backup mode takes AT LEAST the speed of the slow card, but at least in some cases takes the combined (additive) speed of the two cards. What I cannot explain is why when used with the very fast CF card this is no longer true.
I didn't have any fast SD cards, and I scrounged around looking for the older CF cards, not suggesting they are good to use. I wish I had fast SD cards to try, as a lot of people are claiming the faster SD cards write equally fast. I don't think so, For example B&H quotes the UHS-I SDXC 600x Lexar cards as being "min write speed 10mbs".
For what it is worth... someone please do some testing and confirm: