Plastique,
my papers quote a discrepancy threshold of 3.5t for the FU/FOB warning.
Given the number of scenarios in which this warning is inhibited and the fact that monitoring fuel consumption is one of the primary duties of a flight crew en route, I somehow doubt whether this feature is really worth its money.
BTW: The modification consists of a pin programming between FCMCs (9 and on) and FWCs (8 and higher, if I remember right).
Happy Landings,
J.V.