Hi Russ,
Firstly let me say i've just visited your site ........
superb images.
I'm definately going to have to get some digital camera system brochures to know product line up and compatability issues.
In 35mm
film I have used bellows. You are talking extensions of up to nearly 100mm there. Huge magnification, massive light loss at maximum extension, and as you rack the bellows out depth of field drops significantly. Lens to subject distance is also significantly reduced.
My problem here is I do not know if there is such a unit that is compatible with your camera.
If you have access to a dealer stocking used gear you may be able to see for yourself. Unfortunately I am Nikon D100 and don't know if mine will fit ( will have a look though ). D100 does not accept some old Nikon kit.
Just scrolled down and seen wolfies post and as he is a Canon man he should be able to confirm the total magnification available as he has a full set of tubes.
Don