It's not worth a total rewrite of Spell Resistance, Spell Immunity, Dispel Magic, and Spell Breach, in my opinion. There are a lot of moving parts and it gets especially complicated when you start dealing with persistent spells such as Wall of Dispel. While you could do it feasibly well at time of casting, it gets difficult when an effect is being dispelled. There's no reasonable way to determine if a spell came from a potion at a much lower CL, from the caster's spellbook, from another player's item, etc.
All of our magic is already heavily customized, so dropping in someone else's scripts isn't a feasible thing to do. In the case of EMS, I tried to look at the source to at least see what it was doing to achieve its baseline functionality, but EMS only includes compiled scripts with no source, which is just a waste of time.