Sorry Glueball, but there is direct comparison between the 747-8 and 772LRF
I ask you, what has become the staple of passenger long-haul operations? The 777-300ER. It has taken over all the roles of the 747-400. Some of the biggest operators of 747-400 (JAL, ANA, CPA, BAW, SIA) have slowly replaced or are in the process of replacing their fleets with the 773ER.
Even though you would say that they are apples and oranges, it is impossible to ignore the efficiency of the 777 coupled with the capacity of the -300 fuselage. This will only propagate to the freighter market given time.
The only reason the 777LRF is not selling really big is that it is friggin expensive. But give it time, and you will soon see most 4 engine freighters replaced by the big twin.