Intruder....what you say is correct, but you left out part of the story. When you do the increased ZFW you have a corresponding reduction of the MTOW. So for short segmentst, say BLR-SIN where fuel isn't an issue, you do get increased performance. However, for normal segments, HKG-ANC, NRT-ANC the BCF will give you a slight payload increase.
There is another problem with any conversion. That's the floor. On the 744F there's no soft spots, it's all the same floor loading. On the BCF there are some "soft" spots that don't have the dame floor limits as the rest of the main deck.