Here's an UNCROPPED shot taken with a 300mm at only 36 yards (33 meters) - yes, I measured it since it was in my backyard. Same APS-C crop factor camera as your wife's XTi. For size reference, I estimate that coyote was about the size of a 7~8 month old German Shepherd and about 40 pounds.
When I'm out shooting birds with my 500mm, a common comment made by other people goes like "gee, you must be able to get good shots from way out with that thing". So they are naturally surprised when I tell them I must stalk small/medium sized birds to within 20 to 30 feet (not yards) before I can take the shot.