40D + 70-200mm F/4 (IS) + 1.4x TC did not work very well for me and I ended up selling the TC. The problem is that TC's work better with fixed length (Prime) lenses, and on most zooms provide little extra or variable quality. My testing showed little real benefits over upscaling an image taken without a TC. On a 300mm F/4 the TC worked fine, and my 70-200 was as sharp as something very very sharp without one.
On the 40D a 70-200mm lens is equal to 112-320mm on FF. Add a 1.4x TC and you get 157-448mm effective range.