The biggest hurdle for me with the 100-400 was the slow maximum aperture, so I ended up getting the 300 f/3.8 and I use it with the 1.4x extender, faster AF, more light and equivalent IQ. If I'm shooting at dusk, I also have the option of taking the extender off and if I need more reach, then I can use the 2x at a push. While I found the AF on the 100-400 to be quite slow, I did manage ok with birds in flight, including one of the fastest birds (and not that big either), the Eurasian hobby. Sometimes, the 300 can be a bit too fast on the 7D, as it can sometimes be a bit twitchy, depending on the background and focus mode.