Teleconverters are no answer, get get a good sharp image and crop the hell out of a full frame image.
That's all very well, but you can run out of resolution very quickly that way. Crop on of the current 22 MP FF bodies down to match the FoV of a Canon crop, and you are down to only 3000×2000 pixels or so. If you needed to crop the image from the "crop" camera because you were a bit FL limited on that camera, then you could be in real trouble. I now shoot with the Sigma 150-600 on my 50D and still often need to crop significantly while shooting at 600mm. I'll often crop 1/3rd off the length of the edges, so going from 4752×3168 pixels to 3168×2112 pixels. Repeat that crop on the FF sensor and you are down to 2001×1334 pixels, as in each case you are using the same area of the sensor to form the image. I really don't think I am going to get nearly as good an image from the 2001×1334 pixels of the FF sensor, as I am going to get from 3168×2112 from the crop body. The newer bodies are going to give me a few extra pixels here and there with their 18-20 MP sensors.
Oh and to not be FL limited I would need to be shooting a 900mm lens on the crop body, and a massive 1350mm on FF. Unfortunately the subjects are a fixed size, and I cannot get any closer to them, the CAA says so!
So really get an FL that will allow you to fill the frame, or if you can't get the FL, get a higher LINEAR resolution sensor. A good converter on a quality lens should at least allow you to get close to the needed FL while maintaining a decent number of pixels. If I could get a decent 250-1000mm lens, with the handling of the 150-600 I might consider going FF. Or of course there is the 5DS with the same linear resolution as the crop bodies, and that is also a viable solution, although currently out of my price bracket.