I had to do it. At some point I needed to take the 7D out back and shoot Grackles :D
So, for say 145, I made it a challenge. I moved my shooting position farther away, changed the angle to get harsher light, put the teleconverter on the 70-200 and cranked the ISO to 400. So in theory I would have a noisier, more heavily cropped, slightly less sharp version of a shot I already had. I call it "Grackle Picture, version 7D":
Well, I have to say the 7D performed admirably. Pixel to pixel this shot is at least as sharp as the XSi shots were, the extra 6MP really made the cropping no big deal, and even with the teleconverter on saturation is excellent. Noise is much improved as well.
Somehow despite the extra weight of the 7D body, I don't think I'm going to miss the XSi. Also, I can't wait for some other birds to start coming back north.