"Improved AF performance with f/4 instead of f/5.6. The
viewfinder is a full stop brighter, and AF is more accurate; whereas
with a f/5.6 lens, you only have center point AF on most EOS bodies."


Are you sure? Can you point me to a reliable source to support this?


f/2.8 WILL of course improve AF performance when mounted on the right body (one which has the super precision AF sensor at the center). But I honestly don't remember reading anywhere that there is a difference when going down from 5.6 to 4. All the AF sensors in most cameras are sensitive to 5.6 and below.