Ian, what is the larger sensor going to do for you besides give you larger blur?
For reference, I shot this around sunset on Wednesday, at the equivalent of 1200mm. The original was shot in RAW, converted to JPEG, and then reduced in resolution.
You can accomplish a lot with any camera if you have the right lens, you hold it perfectly still, and you have no crap on the front of the lens (fog, fingerprints, etc.).