Hypersmurf said:
Given that a 5th level spell is a minimum of a 9th level caster, you may as well call it -10 and be done with it. There's not much point in saying "The spell reduces SR by 9 points if you're a 9th level caster, or 10 points for everyone else".
Yup, I did actually forget to add "I'd make this a 4th level spell"; if overpowered for a 4th level spell, you could also permit a saving throw every round which, if successful, enables the affected creature to put up its SR again (thereby ending the spell).
I do like the Empowered option, too. Perhaps we can move this to the House Rules board for further enlightenment.
Cheers, Al'Kelhar