If he added a few pounds of muscle he would likely have to change his game more than alot of people believe. Adding let's say 4-5 pounds of muscle would force him to re-learn his shot (simply due to him being stronger) and a few other things. Tone up? Sure. But Bosh
is who he physically is.
I'd more have him work on conditioning (something I think he lacks as he struggles or seems to struggle by April-May) than adding simple bulk to his frame. Also, he should work on defensive positioning more than anything else. He does those few things and he'll be fine and better next season.