Few moments ago I tried doing extreme zooming with my lens that has 260mm maximum focal distance. What I did was using 5x teleconverter which gave me 1300mm focal length. For this kind of photography you will need tripod or reeeeally steady hands, like you are using sniper. Just by blowing on your camera gently you will see camera shaking. What I photograph was Moon, Sun and some regular photo of our road here in my village. You will see one photo where I used only 2x teleconverter and you will see difference.