Harry, I did about 8 forms for the W/D tests, cut way back for the submerse as stated before - I was prejudiced toward Dupla since they were clearly 'way superior in W/Ds. If there had not been the supply interruption with import paired with my wanting to do the article on bio-only canisters, I would not have tested anything else submerse. I got a huge stockpile from when Burleson quit being the Dupla importer for the US and remaindered his warehouse. I did not need any more, but I could not do an article for mainly US audience without alternatives or the technique would have been of little utility.
GF - Yup. That is what totally self-cleaning does for you. And that has a wide safety margin. Remember that I do boost that by another 50% - if they call for that box in 50 gallons, I use a box and a half for 50 gallons. I'm neurotic, but I have not had a biofiltration blip since I started - In the now >12+ years I've been running bio-only canisters. And they need so little attention other than the prefilter rinses.
I pulled one of the fliers from the as-yet unused boxes, they say 3900mm2 per sphere = 29 cm2 each, or about 4.5 sq. in. each. They say that package has 140 spheres (I did not confirm that

), so that would be about 630 sq. in., or ~4 1/3 square feet.
Compared to Ehfisubstrat or other glass media, that is small, but it is IME much more than adequate, in part because it never changes. The old biofims shed well, leaving space for new colonization. The original test Classics are still going strong. I'm using more FilterMaxIIIs as prefilters now, less DIY, but the same functionality. I do still have the original prefiler setups going, not all the original sponges though.