400mm is often to short and I can't afford a super telephoto.
I just got back from 2 days of shooting waterfowl and I am a little frustrated with 400mm. I find it's just too short for soo many shots and a TC really isn't an option since I shoot allot of flight shots and the bare 400 can barely keep up with those.
You can't tell me this lens wouldn't be a huge seller. Any guesses on how much this lens would weigh and cost ?
Come on Canon, forget updating the 400 5.6 and make us a 500 5.6 IS
There are some situations where the 400 is just right but too many times I feel like if I could just be 10 yards closer or have another 100mm it would make the difference in an ok shot to an excellent shot. 



