...If I may ask, do you know how long it takes the buffer to empty on yours?
Wow, that was a bit of a shocker. I was doing some burst tests (haven't shot in this mode for a while) and the camera was slowing down from 6.5 fps after around 8 to 10 shots whether I was in RAW + Large Fine JPEG or even just Medium Fine JPEG.
Did a little research on the Net and it turns out that while I was adjusting settings some time ago I must have enabled "High ISO Noise Reduction" which, as documented in the 40D manual, significantly affects the burst depth even, as it turns out, when shooting at ISO 100.
Turned that back off (I hardly ever use high ISO numbers anyway) and ran some tests. The longest continuous burst I got (at a nominal 6.5 fps) before the shooting rate dropped was in Large Fine JPEG (didn't try lower resolution JPEGs) and ran to 190 shots which is significantly more than the nominal 75 shot capacity of the buffer.
It took the camera over thirty seconds to write that little lot out from the buffer but, of course, you wouldn't need to wait that long before taking more shots if you needed to. With my normal default of RAW + Large Fine JPEG the buffer filled after about 18 shots but the time to empty the buffer was only around 16 seconds.
To finally answer your original question, I did see a post on the Net which claimed that the Extreme IV is around 20% faster at emptying the 40D's buffer so if that is important to you then maybe the extra cost is worthwhile.
The more I learn about the 40D the more awesome it seems.