I own the Lexar Professional 16GB 1000x cards UDMA 7 (150 MBs) and San Disk Extreme Pro 16GB UDMA 6 (90MBs) and have 5D mark III and 7D with the 2.0 firmware upgrade which enhances frame rate for the 7D.
In testing: Both cameras were set at 1/800 f 2.8 using the new 16 -35mm L II lens. Also using camera settings of high speed continuos shooting. AI servo mode with shutter release, being priority over focus, for both first and second images, which will give you highest frame rate. In the field others may be using focus priority or equal priority between focus and release, then frame rate will go down as camera searches for focus longer before releasing shutter. Which means burst will be longer due to lower frame rate.
In testing on 7D, I found no difference in frame rate, or number of shots till buffer is full, between using the Lexar and the Sandisk memory. Shooting RAW I get 8 fps and the buffer is full after 28 shots (3.5 seconds of shooting). Confirms what Canon says that the 7D cannot exploit the UDMA 7 higher transfer rate of the Lexar and will transfer at UDMA 6 rate regardless of using faster rated card.
However on the 5D mark III, I was expecting to see a difference before the buffer was full between using these two cards. The 5D mark III is supposed to exploit UDMA 7 and be able to transfer files at higher rate. In fact I bought a couple of the Lexar cards wanting longer burst rate when shooting birds in flight. Using the Sandisk I got 22 exposures (aprox 3.5 seconds) Well the surprise was on me Lexar got the same fps and burst of 22 exposures. No difference between the two cards. I repeated this experiment several times and got the same results.
Other surprise, the 5D mark III by Canon's spec should only get 13fps with the Sandisk UDMA 6 card and 18 fps with the Lexar UDMA 7 card and I am actually getting 22 fps burst rate for either card.
Based on this test, when using these two cameras there is no benefit in getting the newer cards. I am hoping for the benefit, when they upgrade the firmware for the 5D mark III, like they did for the 7D, which increased the burst and frame rate.
Irony is the 7D gives me same burst until buffer full at 3.5 seconds that the 5D Mark III does. However I get higher frame rate with 7D of 8fps vs the 5D mark III at 6fps.
In the field when Autofocus is likely to be more priority, I am anticipating will be speed of the new 5D mark III autofocus system that will help keep frame rate high while 7D frame rate will drop as it searches longer for focus.
a) Currently no need to upgrade to faster more expensive cards for either 7D or 5D mark III
b) Getting higher burst rate on the 5D mark III than what Canon claims it should using either card.