I've heard from JJ users that when diving head down in a mine shaft that the WOB is noticeable. Perhaps you can add credence?
Well, even though Scuba Client has gotten himself banned, I can now shed some light on this issue. Just completed Advanced Wreck with Dan Dawson at Horizon Divers in Key Largo, and between the course and fun dives after, we spent 8 dives down in the bowels of the Spiegel Grove between 120 and 137 feet in the engine rooms and pump room. Awesome instruction!!
Going headfirst down the narrow trunks two decks in a row (120+ feet), I can confirm a
marked increase in JJ-CCR WOB if your counterlungs are at min volume, or even "ideal volume".
The over-the-shoulder component of the counterlung is where the ADV is situated, and the loop inhalation hose is past that. So if your C/L's are in a low-vol state, most of the gas is in the base of the sacs
above you. With the C/L's flattened in between the gas pocket and your loop, inspiratory effort can be a little frightening, especially if you're space constrained and can't level out for ease of breathing. I
almost shifted to my necklaced B/O in the trunk...
I don't yet have enough experience to say for certain, but the next time I'm there, my plan is to dump some of the wing gas I added with wetsuit compression, and shift it to carry my C/Ls fuller as I approach the trunk. Once neutral at the top of the trunk, I'll see if adding a bit of gas to the counterlung with the MAV to maintain buoyancy over the next 20 feet of descent makes it easier to go vertically head down. I do know I was adding gas via the MAV when things got uncomfortable, but don't yet have a feel for whether I can maintain neutral buoyancy that way in the descent. And with a 2 1/2' wide trunk, I'm glad I didn't have to reach up to my shoulder to trigger the ADV, and could instead just hit the MAV near my lower D-ring with my right hand while running line with my left. Some folks with the older JJ's aren't impressed with the need for a dil MAV, but inside the wreck, I kept my ADV shut off and used the MAV for fine tuning. Head down in that constrained space, it was even more useful.
Frankly, I don't know how JJ-CCR passed CE in that position, unless buoyancy wasn't a concern, and WOB was within standards solely as a result of minimal counterlung vs. lung centroid position difference, with no added resistive component due to a collapsed min volume counterlung. They may have gamed that one, I'm thinking.