(b) The addition of rear fuselage tank fuel impairs longitudinal stability and with this tank full the F.R. Mk. XIV aircraft should not be flown above 15,000ft.
Yes, but the FR XIV had a warning about stability anyway.
In fact, it's a pity you didn't start reading at (a) rather than (b)

(a)On early F XIV aircraft stability about all axes is satisfactory, but on those aircraft with rear-view fuselages, and on FR XIV aircraft, directional stability is reduced.
The normal takeoff weight for an FR XIV is listed as some 380lbs higher than the F XIV. I can't believe a camera installation would weigh that much, but however much it weighed it was situated in the rear fusealge, so greatly reduced stability.
The Spit IX and XVI could take a 75 gallon rear tank. Again the manual says by special order only, but later in the manual it says no aerobatics with more than 30 gallons in the rear tank, whereas the RAF manual for the Mustang says no aerobatics with any fuel in the rear tank.
As the manuals seem to have slightly more warnings about the Mustang with rear tank than Spit IX with rear tank, I honestly think the "by special order only" thing is a reflection of training, rather than any inherent danger.
Mustang pilots would presumably be trained to fly with the extra tank, but as it was rarely used on the Spit, I doubt many Spit pilots were. Being used to flying a more forgiving plane like the Spit, the extra difficulties caused by the fusealge tank must have been even more dramatic than they were in the Mustang.