I like it, but maybe, instead of +10% per enhancement bonus, you should make it +5%. that way, it's gotta be a +5, with five more effective enhancement bonuses from various other effects (thundering, shocking, etc) before it becomes 100% vs. incorporeal... or maybe even 4%, which would still give the ghost a 10% chance against the +5 Mace of Thundering, Shocking Burst, and Disruption.
I mean, with spells, there's still a 5% chance that a 9th level spell will miss/fail. And even then I'd say that spells designed to target Undead specifically should get a +5% (so Disrupt Undead would confer a 5% bonus to hit an incorporeal creature)