The best documented WOB of any rebreather is 1.44J/L at 40m on Air at 75lpm
This rebreather is a BMCL unit. It also has its 3D CLs as close to the divers back as possible as the units frame is the backplate with the harness built in. It doesn't have an optimised position because it was designed to work in the full range of positions.
Testing at
http://www.deeplife.co.uk/or_files/DV_OR_WOB_Respiratory_C1_101111.pdf
That said there are also BMCL units inclusive of the KISS and Drager designs with extremely high WOB but this is also reflected in their inability to be CE'd. Balanced by there being a lot of FMCL designs that barely scrape through the CE minimums of 2.75J/L when 40 year old MK16's offer much much lower WOB.
If the set be it BMCL or FMCL is fitted correctly on the diver, then the WOB ought not to change from that tested. The manikin used to test WOB is not that much different from the average diver. Which Dave is I think where your statement has a fallacy, in that you have assumed a FMCL may offer better WOB over BMCL! Certainly possible but by no means a certainty.
Obese divers might have issues in experiencing increased WOB, due to these CL's being further displaced from the lung centroid but I don't think anyone has had sufficient interest to investigate that as a factor. This would equally affect FM as well as BM CLs.
It would certainly be interesting being in the OP's position and being able to directly compare the WOB of his set based on the manufacturers testing of it with optional BOV fitted with both BMCL and FMCL options. Pending knowledge of what the actual WOB is for both configurations any discussion is purely theoretical anyway.
SR, I'd agree with you based on my own experience on certain units. Folk are certainly re-experiencing these issues with untested sidemount units
My Journey Into Sidemount Rebreathers but where BMCL are designed for optimised performance, the evidence
http://www.deeplife.co.uk/or_files/DV_DLOR_HydroImbal_101116.pdf doesn't agree with your experience to date.
Compared to the expense of designing and testing BMCL to ensure they actually have low WOB, OTSCL are cheap!