A 2x teleconverter will degrade the image quality of the photo, which would be bad for the 75-300 because it's already the worst telephoto zoom lens made by Canon, by quite a ways. Also, if you add a teleconverter and the largest aperture is above F5.6 then the Camera loses autofocus. Since your lens is F5.6 to begin with then adding any teleconverter will lose autofocus.
Does anyone have experience using 2x1 tele converters? I'm considering using one to extend my Canon 75-300mm lens that I use with my Digital Rebel.