what kind of a camera would you be using a 300-800mm lens on? I ask, because on any camera that is not a full frame camera, that might be too much lens. Too much lens from a size / weight standpoint, but also in length. I mean, that lens is a doozy. Big time.
Let's put it this way. I shoot with a 10D and a 1D, and use a 300mm f/2.8 IS for my long lens, and use a 1.4xII teleconverter off and on. I just don't think you need that kind of a lens. You would be better off with a 500mm lens and a 1.4xII TC. That would give you an f/5.6 lens, and would be very sharp.
I am not aware as to the optical qualities of the 300-800mm, but I doubt it is as good as a 500mm prime + 1.4x teleconverter.
Just my $.02