Setting Difficulty Offset to its max of 1.0 only gets you level 120 dinos. If you want level 150 dinos, you need to set the Override Official Difficulty setting to 5.0.
Changing the difficulty does not cause all of your existing dinos to change their level. It only affects new ones that spawn. If you want all dinos to have the new level, you'll need to kill the existing ones with the DestroyWildDinos admin command. You can learn more about using admin commands here.
If you want to learn more about Ark's very complicated difficulty configuration, read this: http://ark.gamepedia.com/Difficulty