The High ISO Noise Reduction (which is actually active at all ISO settings) on the Canon cameras will drop the camera's ability to buffer photos drastically. I shoot some basketball games in some poorly lit high school gyms and rather than limit myself to single shots with a flash I pump up the ISO to 4000 or 5000 most nights. I shoot with Noise Reduction set to high and I can only shoot four photos in a burst with my 7d.
You were bang on. I set High ISO Noise Reduction to disabled and was unable to fill the buffer shooting continuously.... I gave up after 150 shots.
My earlier observations showed that it had to be a computing power problem. If it was a card wrtite speed problem changing the raw file settings would have sped up the rate, but it had no effect... Likewise, if it was a write speed problem, why would 8 tiny jpegs fill the buffer quicker than 16 large raws?
Thanks for the tip.... I had assumed that high ISO reduction would only affect high ISO shots, not everything.... and I was wrong.