So using the picture below as a test subject, I setup the camera with identical settings. Fairly fresh battery, 28-135mm lens at 135mm, F/5.6 at 1/500 second (one stop underexposed - see note below), manual sunny white balance, manual focus and no IS on the lens. All Custom Functions are default, especially noise reduction CFII.2 which dramatically reduces your burst depth.
I then basically just held the button down while the machine-gun fired at 6.5 frames/second. After 4 frames of reduced burst shooting, I stopped. I also timed how long it took the images to write from when I first started. I repeated the test twice with each card and the results were almost identical as can be seen. Note that since the image was underexposed and had a lot of similar tones, more frames were possible since not as much JPEG compression was required. But I figured this was a good way to highlight the difference in CF card speeds.
Extreme III: 143 & 147 shots - 54 & 56 seconds to write - 5.5 MBytes/second
Ultra II: 124 & 126 shots - 63 & 65 seconds to write - 4.1 MBytes/second
Again, don't expect this many shots in normal shooting since the underexposure and similar tones yield files about half as big as usual. However, the data indicates that the Extreme III is about 35% faster ... although in real life, you would rarely be shooting this many shots continuously. Remember that once the camera has written some of the data to the CF card, you can continue shooting while it continues to drain its buffer. For those wondering, when I use thed Canon 40D's USB interface to upload to my PC, both cards transferred data at about 4 MBytes/second ... but the limiting factor may be my USB connection which is over 10 feet long and goes through a hub and then my monitor.
Don't expect me to do this test too many times ... the shutter is
"only" rated for 100,000 cycles ... so I just burned through
almost 1% of my camera life! ;-)