Sorry SR, I've only just seen your post. With proper engineering, BMCL
will breathe better across the full spectrum of unmanned test requirements than OTS. It's not a guess. Given suitably indepth unmanned testing eventually being been done on OTS units and the results (with calibration) published, anyone can readily do this comparison against that offered on the Apoc.
Deep Life Design Team: Selected Design Validation Reports for DL & Open Safety Equipment Ltd's Rebreathers
OSEL even sell a breathing simulator to make this R&D easier for rebreather manufacturers, so they can offer improved products with better breathing performance.
http://www.opensafety.co.uk/files/Datasheet_iBreatheMkIV_1906.pdf
In response to your specific question, note the shape and material used in the Apoc IV & II counterlungs. Some of this is disclosed in the FMECA
http://www.deeplife.co.uk/or_files/FMECA_OR_V4_140831.pdf
You won't get the high breathing performance offered if you replaced the CLs with MSR waterbags or added a SS backplate and wing into the equation. This is why the design (noting the Apoc is a spin-off from the 350m rated commercial saturation diving primary life support system that DL was originally commissioned to design) started from a clean sheet of paper, with money not being a factor, and anything that didn't improve performance (or hindered user safety) was critically examined.
I'm in full agreement with you on the importance of WOB. But can't see OSEL offering OTS unless a military client has sufficient mission justification for the need and accepts the decrease in breathing performance. In which case, like with the various Incursion models OSEL is shipping, this wouldn't be offered to recreational divers.
You'll obviously improve the PRISMs WOB with fitment of the ALVBOV, which should even subjectively, be noticeable; however, I don't recall that the USN published WOB for the PRISM (don't think SM/PR tested it?) and without that we can't discuss just how much of a WOB improvement you'd see swapping to an Apoc. Still like to see one fitted with analogue secondary and modern primary HUD though.
Under the current 14143 standard there is an elastance limit, a breathing resistance limit and a work of breathing limit. There are also hydrostatic limits. The elastance, and hydrostatic limits are related but independent of WOB.