It's not the camera that's the problem - it's your kit lenses. When you use a TC 1.4, 1.7 or 2, you will lose some light and hence some speed. So for example, when I use a 2x TC on my 70-200 2.8, well I lose the 2.8 speed and it drops down to no faster than f/5.6. Your kit lenses are already very slow so adding a TC will make them essentially unusable. In fact, I doubt the AF system would even operate with a TC mounted to a kit lens.