As Dave said, the f/4 compares very favorably to the f/2.8. Here is a small gallery with some comparisons mostly of the 300mm f/4 & f/2.8 versions, with and without TCs. I was a bit surprised just how well the f/4 compared as the f/2.8 is one of Canon's legendary lenses in terms of image quality. I use them both, but if I don't need f/2.8, or the 2.0x TC, the f/4 is my first choice as it's smaller, lighter and more compact. I can carry a body, the 300 f/4 and another lens in a shoulder bag, I need a backpack with the f/2.8. The f/4 is easier to handhold, while I need a monopod after awhile with the f/2.8.
http://www.pbase.com/dougj/lens_tests