I have two suggestions if you want more reach:
1. Get closer
2. 7D
Don't go for the Sigma 50-500. In general that kind of lens just means trouble IMO. Especially for $1600. I know people who buy a 50-500 or Tamron 200-500 so they have more reach than their 300 f.4 or 100-400 and then realize they prefer using the Canon L lens anyway.
The 7D will give you more reach, but for $1700. It would be a nice advantage to have both a full-frame for portraits and a crop body for action, but it is possible you could just save $1700 by getting closer. If you want more reach for birds (usually the case), a blind, LensCoat, and some practice can save you the expense of buying more gear. Of course, sometimes more reach is needed. Could you specify why you are looking for more reach?
my 2¢
brendan