Under performing would be failing to meet a dps check in the content your doing it in. underperforming is relative to the content you are playing.
I believe there is no such thing as over performing and heres why. We all have to run dungeons at some point in the game and over and over for our tomes for gear progression. Now square enix has been and always will make some of the tome gear BIS for there gear cycles which is every 2 patch cycles. Where am i going with this? Ultimate players have to farm casual dungeons of course they are going to blaze through the content.

since they are forced to do such this means dungeons were built with everybody in mind. So you cant over perform ina dungeon because your top end players have to do the dungeons anyways

People can get angry at me for saying this all they want but over performing isn't a thing