Is sunder useful if infinity breaks the resistance?

Besides those exceptions, does it stack? Does sunder help?

To my knowledge conviction stacks with sunder, but treats the mob as if it was still LI (i.e. only at 1/5th effectiveness)

Example: LI mob with 115 resistance
Without sunder: Conv lvl 12 is -85%, so you still get -17% from the aura -> Mob ends up broken at 98% LR
With sunder: Sunder breaks LI and sets LR to 95%, and then conv works at 1/5th -> 95%-17% = 78% LR
Note that direct -light res like griffon, facets or holding infi yourself still work at 100% effectiveness on broken mobs

So aside from breaking all LI, it also gives you max(0,LR-95%) light pen

Ciras is correct. With sunder every mob that was immune get res lowered at 1/5 of conv aura