Since I'm loaning out my EOS 7D I was using my 5D Mark III and, without the crop factor of the 7D, I used a 2X Extender with my 70-200mm f/2.8 lens. I used this setup for a daytime match last year but I wasn't really satisfied with the image quality this time. In addition to the focusing being slower with the extender, even in focus shots displayed more softness than I like. But unless your looking at full sized images at 100% you probably wouldn't notice.
Here are some examples:
|400mm, f/5.6, 1/2000 sec., ISO 800|
|400mm, f/5.6, 1/1500 sec., ISO 800|
|380mm, f/5.6, 1/2000 sec., ISO 800|
|260mm, f/5.6, 1/2000 sec., ISO 800|
|400mm, f/5.6, 1/3000 sec., ISO 800|