Imoen is a terrible thief but she can become a mage capable of using level 8 spells, She also can reach the exp cap before finishing the game unlike what Neb said.
I suggest you lose the Sorcerer as even thought they can cast six of each spell they know and take Imoen, she can know all the spells so they can be changed to fit any situation and if she runs out of magic you can give her the a ranged weapon like another Bow.
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Hodge-podge Sliver Cleric of the ALSB
You don't need pants for the victory dance becuase I am better then Weasels!!