Hey mate,
On your camera, the 200mm lens will yield 200x1.6 field of view equivalent. That is, 320mm.
If you bang on a 1.4x extender, that will take you to approximately 450mm, at f/4.
It won't destroy your ability to focus at infinity -- autofocus will work. I can't remember if I have put the 1.4x II extender on my 200mm f/2.8L II USM before, but, I have definitely with the 70-200mm f/2.8L II USM, and it works just fine.
Here's one from four years ago that I shot with the 200mm and a modified 40D. I can't find a link to a bigger version of it, but, this was stopped down to f/4 (hence the diffraction spikes):
https://lh6.googleusercontent.com/-V...d_Orion_GP.jpg
H