From what I can see from the Boeing documentation, the difference in default CG (4% FAA - 18.5% JAA), approximates a +/- 2-tonne (!) difference in aircraft mass when the FMS calculates max FL. (the FMC adjusts total a/c mass to compensate for more/less tail down force). Hence, we always take the load sheet take-off CG and subtract 4%, then put this number into the FMC. Usually ends up around 12-16% on our B737-300.