Taking the most efficient choice isn't system mastery. System mastery is basically breaking the system while staying with in the rules.
No... That's one possible (and highly annoying)
use of "system mastery". You seem to be applying your own definitions and biases to terms like powergaming and system mastery, and then saying that we're incorrect because those words don't mean to us what they mean to you.
My definition of system mastery is knowing all the rules and mechanical elements of the game and how they interact as they're written.
Any good player should strive for system mastery even of a beer-and-pretzels game simply as a courtesy to the other players as part of the social contract of the gaming group - if you can't be bothered to learn what your character's abilities do and hold up the game for five minutes every time your turn comes around, you're being rude and inconsiderate.
Powergaming involves then using that knowledge of the game rules and mechanics, and the synergy between various elements of such, to best advantage. What you choose to
do with your knowledge is the actual crux of the issue you seem to feel so strongly about. You can "powergame" towards any goal you choose, and this is not an inherently disruptive behavior. It only becomes disruptive, just like anything else, when the person choosing to do so does it in a vacuum without any consideration for the tone of the game and the desires of the DM and other players. Do you believe that optimizing for maximum damage is the only way to powergame? I can build a Life cleric maxed out on healing, and contribute just as much damage to the fight as the raging barbarian by keeping him on his feet killing things
long after he should have gone down. On the other hand, I could use that same maxed-out Life cleric to keep a party of "Jimmy's special little snowflake" characters from getting TPKed when they happen to stumble into a fight they're nowhere close to being able to win (because they didn't bother to devote any character resources at all to making sure their character
survived long enough to make their fancy speech to the King and talk him out of starting a war by rolling some dice)...
Would it be more or less disruptive if the party bard managed to talk everyone he met into joining his side of things? Sucks that your adventure has nowhere to go because the Bard turned the mob of angry protesters into a community project to pick up all the trash in the town square.
Bringing a raging death-o-matic barbarian to an espionage game is not powergaming, because you've just gamed yourself right out of contributing anything to the party since there's nothing around to hit. Bringing that same maxed-out barbarian to a game where the party is a bunch of average Joes isn't powergaming. All you've done is made the rest of the group unhappy and possibly gotten yourself disinvited from the next session. If, as you say, "powergaming" means that winning at all costs is the only thing that matters, then you've lost.
Your problem isn't with powergaming or powergamers, it's with self-absorbed idiots who don't care whether or not their idea of fun is incompatible with the rest of the group, and powergaming for big numbers when rolling dice is something that produces immediate gratification without having to invest great effort into it. Particularly in combat. Combat requires more rules than other areas of the game, and thus there's more chance to use those rules. Which makes it a very attractive way to get their "I win" fix.
