In most cases the FMS is what 1980's technology? With a tiny amount of processesing power... Verses a flight planning system running on something much faster with a full worldwide set of GRIB winds.
As CONF mentioned its important to A, check how old the OFP is and B, to make sure they used the correct set of wind(s) for the flight.
In the current aircraft an OFP will be provide the best results I think, moving to the future we will see more EFB based applications to assist with tactical reroute and level optimization.