Congrats on the new toys Tom. Like I mentioned , the 70-200 f4 is a stellar lens. Don't let the price or the fact it is not one of the more expensive versions fool you. It produces top notch photos.
I have been wanting some of the Kenko extension tubes myself so that I can use them with my 400mm L and be able to get a closer focusing distance. The nearest I can get right now is 10 1/2 feet
Keep us up to date as you get to play with them more and more.