!! impressive setup Paul! Out of interst is all that tape around the flashmount area just to help hold the angle mount steady? I have tried similar setups with my 580M2 and found that the weight of the flash tends to pull most small heads downwards (ballheads are no use at all).
As for extension tubes they are for mostly macro use - giving you a reduced minimum focusing distance so that you can get closer, at the cost that you will lose focsuing on further off subjects (a few feet to only a few inches depending on the lens and tubes). Also the amount of increase you get in magnification is proportional to the focal length of the lens and the tube length - the longer the lens the more tube length you will need to get to a fixed magnification point.
What does increase your focal length and will give you closer shots of further off subjects is a teleconverter (or extender as canon call them to be difficult) however its important to note that your image quality will take a noticable hit with one of these, especialy on a lens such as yours with a very long zoom range. Personaly I would say that you are likly toget better shots from cropping than you would from using a teleconveter - also canon brand ones will not fit your current lens - they only fit onto select L grade lenses. Kenko and Sigma make their own teleconverters which get very good reviews, though I have no idea if they will fit your lens either