Of course, a character can be expected to seek the most effective way to acheive a goal within his range of competence and within the parameters of his objectives and personality. In that sense, his going the most effective route would be expected (and in that case would not be the antithesis of good storytelling), but that doesn't mean that powergaming per se is a neccessary ingredient in great storytelling.
Powergaming is (IMHO) by definition when you are playing to win as opposed to creating a story - specifically when you the player are attempting to win the game as opposed to creating a story about the character attempting to acheive his goals, as viewed from an in-universe meta-free perspective. This may or may not be consistent with what would be done by a character if the goal was good storytelling, depending on the situation. These are two seperate objectives which may well coincide, but often do not.
I guess the issue is partially whether you are out to create a character with believable goals and personality and with complex desires and aspirations beyond "kill all the monsters in the dungeon in the most efficient manner possible". That, and whether your character's profile reads like the description of a Mary Sue or of a plausible person with both strengths and weaknesses that the player acknowledges and honors during play.