I just returned from 3 wks in Zambia with my new 40d. My most used combo was the 300 F/4.0 L IS with the 1.4 tc. the 40d focused really well, much better than my EOS 3's ever did, even in low light (spotlight on animals a fair distance away, no flash).
I also had a 70-200mm f/4.0 L IS that I used with and without a tc. It's a great mid-range lens, focuses really fast, and its IS is really effective. I don't have a backup digital SLR as yet (used a P&S for grab shots and walking around in public),, plus there were occasions when the wildlife was so close I needed to use a shorter lens. So I did do some lens changing but would guess around 75% of my photos were with the 300/tc combo and was quite happy with it. The sensor cleaning system really works. The only dust spots I ever saw on an mage were early on when I left the camera on for a long period of time, so the device didn't activate.