By this standard, someone who refuses to full sac, multikill, and gang his enemies is a powergamer:
Most conventional RP would call for you to see that your enemies lack proper equipment, and to see that they are dead as efficiently and as quickly as possible. However, I would consider a player who refuses to engage in this kind of behavior a fairly decent player, and not a powergamer at all.
While I do think sometimes powergamers ignore RP, more often powergamers will use their RP to justify their douchebaggery - he was my enemy so its ok to sacrifice all the items I can't use. My character hates magic so its ok to strip this corpse because I need to make the veil thicker. Me and this guy are sworn enemies so me waiting at his pit with a tuned lyre and a sing fiend trigger for when he appears in the room is simply good tactics.
Etc.
I think powergaming has very little to do with RP, to be honest. There are situations in which following good RP is the powergamey route, and engaging in questionable RP is the good sportsmanship route.
