FPOBN,
One could argue that the UK had already done 'RECAT', with its designation of:
Heavy
Upper-Medium
Lower-Medium
Small
Light
Just swap the words 'Upper Medium' for 'Lower Heavy', or Cat C in RECAT1A parlance.
Rather than ICAO...
Heavy
Medium
Light
Also, your chart of the FedEx 777 wake turbulence separations.....
From the FedEx document you reference, the runways they use do not currently have the 2.5nm waiver, so the min wake sep of 3nm is illustrated...
(note 777 to 777)
Under RECAT1A, 777v777 is 3nm for
wake separation. It is
not dependent on radar separation rules as you say. Even if they did operate into a US runway where the separation
could be 2.5nm (I'll take your word for it that they don't) then it would still be 3nm 777v777.