I went to Farnborough with a 7D, 70-200II and 2xIII, I kept the TC on for 60% of the day I think. Didn't need it with the Vulcan. Whilst I did occasionally want more reach, it was only the smallest of planes (WW1 battle recreation) and when the Harrier was hovering (I was just too far down one end of the runway). With things like the Red Arrows, some of the wider shots I took were at 70mm to get more of the overall formation in, I don't know if you shoot that type of shot, but its worth considering.
I'd have thought you could crop from 400 on the 7D to get the equivalent of the 40D + Sigma combo without too much loss of quality? Would save on carrying an extra lens/camera!
Wish I could go to this but I'm working sadly. Going to Clacton on Sea on thursday though, looking forward to the Vulcan again (who doesn't
) but I wish they could have got the P51, that put on a great performance last year!