You conducted a very interesting experiment. I also think there is a great deal of physiological variability when people undergo extended levels of muscular exertion/CV stress. Personally, while swimming hard I have not noticed an urge to suck into the mask. Why, I don't know. I do tend to get cross-eyed though. But you've got me thinking that perhaps there is some brain-spatial relationship here?
With regards to the rebreather maintaining a rhythm of breathing is very, very important. These days I pace myself as much as possible. If I breathe hard on the loop I try to stop before a cycle of hard breathing begins. It is all too easy to get into. Pushing a human body loaded with heavy gear, draggy suit through a dense fluid medium is asking a lot from the muscles and heart. Add current and stress - Whoa! Additionally, human CO2 retention (large variable in my opinion) as well as the scrubbers ability to remove CO2 would be factors in addition to age of sorb, scrubber design, depth etc. so best to take it easy.
A smooth bore snorkel would most likely have less WOB than a loop where you physically have to move the gas around around paths and hose corrugations. However, if you had one of those wacky snorkels with a ball it would arguably be like breathing through a straw.
