I think the sweet spot is aroung level 30. Player can do a lot even on max difficulty but he / she is not overpowered. I did several gameplays with different builds and game became less and less chalenge from level 30. On the other hands, if you limit yourself to be non-leathal, takedown enemies only when they discover you and be stealthy, its still fun even on higher levels. I have recently stopped with to much optimised builds and make more jack-of-all-trades builds to have different options and not stuck with just one overpowered gameplay.