I'm just wondering if anyone has tried stacking their Mk III extenders with the 12mm extension tube and attaching that to a Mk II super telephoto. I realize it would compromise image quality considerably, but I have all the parts other than the 12mm extension tube, and could see myself using it in some situations. I ask because I see George Lepp using similar combos with good results (see below) and wonder if it's worth getting the 12mm tube to use the extenders with my 300 2.8 IS II. I plan to use it for its intended purpose, but was curious about stacked TC results, too.

http://www.outdoorphotographer.com/how-to/shooting/extreme-close-up.html