H@rry, it is not the tubes that you are disappointed with. ;)
And, for web purposes, you are probably right, you may get the same results with a crop in post processing, but for actual prints, you would notice a HUGE difference.
That macro lens would be an excellent choice, and the tubes will become of much more use.
Oh, just for reference, the coral I used in this example is about 9-10" away form the front glass.