Some people have reported that the difficulty settings don't change the spell resistance on enemies. I'd need to test it, but if true it'd put the "just lower the difficulty" into question if it is your spellcasters who are struggling to land a spell.
A pity you do not have spells that ignore enemy spell resistance in the videogame or feats and race or class features to improve your chances to ... hey wait!
PD: I checked and you were right, the Spell resistance is static despite your game options. The other stats of the enemies, number and frequency, the abilities they use, etc you can change.
