Hey Ken,
I don't know if it's different on a 10D, but I did try a Lexar 40x on my 300D a while back. I filled the camera's buffer until it was full of 8 compressed jpeg images, and then I timed it writing that data to the card. The Lexar 40x took about 20 sec to write all information to the card whereas a Sandisk (I think) 1x took about 30 sec to write all information to the card. I ran the test a few times and got consistent resuts.
I don't know how those numbers are going to differ for the 10D, but maybe that will give you a basic idea of the performance gain with the 40x card. In the case of the 300D, I shaved about 33% off my write times. I hope that is helpful. 
Take care,
-Deva