personaly i think the pixie is overpowered anyway, between the DR, spell use, theif skills, and sneak attack it's quite posibly the best of all the familars (arguable mostly due to the mephits regen and elemental attacks)
i had thought though that bioware standard the pixies were stuck at 1d6 sneak attack no mater the level, mostly because with stronger sneak attacks they become overbearingly powerful and can easily be used as back up fighters