Spell Penetration applies to mitigation or spell resist value?

joshisanonymous

What I mean is, is there a benefit to going over the cap in spell resistance in 1.6? If penetration applies to the total spell resist value, the actual number, does that mean that going over the cap could potentially mean you still have full mitigation against some spell penetration users? Or is spell penetration a direct reduction in the mitigation percentage of the person being attacked, no matter how overcapped they are?