The Invulnerable Rager Barbarian archetype also specifically applies his DR at 2X base value against nonlethal. That's the only example I know of in which DR on its own is specifically more effective against non-lethal (barring the hypothetical existence of a creature with DR/lethal)
Depends on typing. DR only applies to 'physical' attacks; piercing, slashing, and bludgeoning. Pain Strike is not typed as one of these, so it's my understanding that DR doesn't help against it.