I'd call that unacceptably soft. Given your shutter speeds, it's unlikely to be motion blur or camera shake (especially the second shot at 1/640 s). Could be the AF, could be the lens. I'd suggest testing with Live View and manual focus (or autofocus in Live View, as long as you're not using the quick mode where the mirror flips up) and see if that's sharp, as elflord suggests. If that works, you need AFMA on your 7D (XSi doesn't support it). If it's still just as soft, I'd contact Canon Service.