As noted above the actual capacity is going to vary a bit - not so much from the bladder, but from the cover over the bladder.
The bladder itself is *probably* RF welded using a pre-made die so there will be very little variation from bladder to bladder. The sewn cover however could introduce some variation due to variation in seam locations, excess material inside, etc.
However the major issue is how the wing operates with the rest of your system.
For example, if your plate is a little flatter than the one the wing was tested with and/or your tanks are larger in diameter, have boots on them, or is perhaps you carry an inflation bottle or some other piece of hardware that prevents full inflation of the wing, then the wing will not carry the rated lift. Sometimes the hit you take is severe.
Sidemount adds another element as bungess over the top of the wing and any wing control devices will reduce the actual lift. For example the Nomad went from using a Trek wing to a Rec wing to it's own even larger wing to accomodate the losses due to bungees and wing control hardware.
---
Looking at it differently, fresh water weighs 62.4 pounds per cubic foot and salt water weighs 64 pounds per cubic foot (on average - salinity varies).
So if you have a 55 pound wing (in salt water), the lift in fresh water would *only* be about 53.6 pounds - a loss of 1.4 pounds compared to salt water.