The text of Ultimate Fiendish Form reads: "You gain Spell Resistance equal to your Charisma score."
Testing on one character with unbuffed charisma of 100, my spell resistance is 81.
If I enter a quest on reaper and take a yugo potion my charisma goes to 106, but my spell resistance remains at 81.
If I equip and unequip gear my spell resistance remains at 81.
Equipping Shield of the Scorpion raises my spell resistance to 89, which is +8 for insightful spell resistance, but doesn't recompute the total spell resistance.
So I then reset my enhancement tree on my ship without reaper bonuses, but with yugo potion, bard buffs and +alchemical orb with +2 charisma and my spell resistance is permanently reset to 106.
So, it appears that once you select the capstone the spell resistance is locked in at your current charisma.
So reset it at cap and max out your buffs!