I do not think you can make a bouyancy estimate base on the difference in travel weight between an ExpTech and another brand BC, if I understand your question correctly.
You could preload your Exp Tech with a 1 pound lead bullet weight on each side of the waist strap and just travel with it.
If you had an OOA emergency at the surface that you could not deal with, 2 pounds likley will not make any difference anyway.
You could just ditch your somewhat negative camera.
We have all known photographers that would rather drown than loose their system.
You could preload your Exp Tech with a 1 pound lead bullet weight on each side of the waist strap and just travel with it.
If you had an OOA emergency at the surface that you could not deal with, 2 pounds likley will not make any difference anyway.
You could just ditch your somewhat negative camera.
We have all known photographers that would rather drown than loose their system.