My f3 forks are fitted with Shimano XT v brakes , so in effect the 'rotor' size is around 26" in diameter !
The max load I can put through the fork occurs when the wheel is on the point of locking up . This stopping
force is applied to the bike through the front axle and is transmitted up the suspension legs via the suspension linkage
to the headtube . I don't think that the rotor diameter can have any effect on this stopping force .
However there is the question of the max bending moment applied to the fork lower leg when the caliper is being
'twisted' forward on the point of locking up . This problem is more often seen on rear disc conversions where the
caliper is mounted mainly on the seat stay leading to considerable flex in that area . The f3 lower leg is already
of quite generous proportions and comes fitted with disc mounts so I think that a change in rotor diameter
is unlikely to be a problem . Hope this helps . Have you got access to any AMP spares ?