There's really only two ways to get closer, move the camera, or lengthen the lens. If you were to spring for the 300APO then you might consider the extender to solve the length issue. I don't think I would use the extender on the 500mm, but then if I had it I would probably try it.
Depending on how you get to where you are wanting to use that lens, the weight might be a factor, it is rather heavy. If lesser weight is required you might want to think about the Mamiya mirror 500mm.
The 500 5.6 is one of those things that if you use it to grab an image that you could never have taken with a shorter lens by moving closer, it would suddenly be worth all the effort/expense.