Bit late on this thread but here is my take on the 300/2.8. After using the 400/5.6 for three years I recently decided to upgrade to the Canon 300/2.8.
I was confident that the IQ with a 1.4tc would be great but was not so sure how it would pan out with a 2x tc. I have to say that I am over the moon with the performance on my 40D. With the 2x attached AF is a bit slower but adequate - using the focus limiter helps a lot.
Attached are a few shots taken at 600mm with the 300/2.8
edit: how the heck do you show the image in the thread?
