What was so immersion breaking about him? He seemed to react to his surrounding in an appropriate manner, and knowing nothing about DnD, I always assumed Boo to be a normal hamster, whom simple-minded Minsc imagines to be something more. The closes thing to immersion breaking I can think of, are things characters say when you click on them - "Yes, oh omnipresent authority figure?" - no doubt a left over of RTS genre, and not the best fit for BGs. And definitely there were things I couldn't stand - like the taking sword.
What I find funny about Minsc is that getting him out of the cage is basically about getting him angry enough to bend the bars. And yet the guy had a frontline view on the main character being tortured by Irenicus, and somehow this didn't make him angry at all.
