luchtvaarnieuws.nl writes the following (summarised)
The B747-8F uses 2.7% more fuel then was promised. This was cocnluded after the variety of test flights. Cargolux already agreed on a compensation in June but since Qatar holds 35% of CLX they rejected that offer just before delivery. They now want another compensation from Boeing and from GE. GE is now working a PIP (performance increase package) for the GENX-2B. This reduces the higher fuel usage to 1.1%. Problem is that this package will be available at earliest at the 3rd quarter of 2012. Boeing and GE are also working together to modifications to increase the performance of the B747-8F
source (in Dutch):
Article - Luchtvaartnieuws