Currently, Cargolux, Nippon Cargo, Atlas Air, Volga-Dnepr Airlines, Korean Air Cargo, and Cathay Pacific Cargo are the only operators of the Boeing 747-8F.
Also Saudi Arabian Cargo.
I believe the -8F would benefit FedEx. It could even help replace some A300s, A310s, MD-10s and MD-11s. I believe it would benefit because it is more efficient, double the range of the A300, A310, ranges about 1,000-2,000 km longer than the MD-10-10 MD-11, but doesn't range as far as the MD-10-30 (MD-10-30 ranges 10,622 km while the -8F ranges 8,130 km). The MTOW is triple the MTOW of the A300 and A310, and 300,000-400,000 lbs more than the MD-10 and MD-11.
Do you think FedEx would benefit with the -8F? Would they order them in the near future even though they have 50 Boeing 767-300Fs and 20 more 777-200Fs on order? If the A380F would've existed, would they have benefited with it? Would UPS have benefited with the A380F and/or the -8F?
I think FedEx could benfit from the 747-8F if they used it to replace smaller freighters on their most popular routes (maybe LAX-MEM; I'm not positive, but that is probably a popular route for them). Also know that smaller airports need smaller planes, so on lots of routes small planes replaced by small planes, not large planes (except when demand rises).
Also, the 767-300Fs are coming in to replace the DC-10Fs and maybe the A310Fs.
UPS might've benefited from the A380F, but I would only see them ordering that to compete with FedEx or if demand rises. The 747-8F could benefit them in the same way I mentioned it could benefit FedEx.