Actually, it would turn your 200mm f/2.8 into the equivalent of a 600mm f/8.4. The intensity of light per area is still f/2.8, of course, but with 9 times less area, the total amount of light is 9 times less (3 stops). Furthermore, the noise will be three stops worse (like ISO 12800 instead of ISO 1600, even when ISO is kept the same). Also DOF and diffraction will be like a 600mm f/8.4 lens. I did a thread that covered this recently.