If it’s 90-something when your performance occurs, it’s likely that the organ will be sharp (pipes go sharp in warm weather, flat in cold), and the amount of sharpness can vary greatly based on many factors (how close the organ chambers are to the outside, the temperature of the air going into the pipes, etc.).
That sounds like a good idea to scope out the organ’s pitch the day prior to the performance, Brook. Take your chromatic tuner, have the organist play using the registration he or she will set for the Grandjany, and see what your tuner reads for various notes in the key at strategic points in the piece. Best of luck to you, Sidney.
The tension would be increased by 2.93%. But A-465 is more like a semitone, which means 11.7% higher tension. I agree: see what the luthiers and technicians say about it. Where are you located, that organ pitch can be that high? Holland? (thinking of the 4-manual Schitger in Zwolle)