>not been my experience as the camera writes to the card at a >much lower rate than even a class 4 can accept.
No, certainly not class 4. I recall pretty clearly that I obtained a benefit in in-camera buffer clearing speed, right up to 45mbps cards. After that yes, there is no improvement (but that speed is well beyond Class 4 and 6 and beyond many early Class 10 cards too).
For me that's what this thread is about, the cutoff point.