You ask about boxing helping pitching--- I don't think it would hurt you any. A lot of "explosive power" as you call it , in pitching is generated from the legs and lower abdominals, if your boxing , then your doing crunch's, jumping rope and a lot more leg and ab strengthening with pushup's ,setup's etc. So as far as lower body strength I'd bet you got it. The arm speed in boxing and in pitching are different , like in boxing you punch straight to the mark.
In pitching your actually, sort of 'whipping' the arm through in a sort of arc, so this part I'm not sure if boxing helps, I doubt it, But as far as overall conditioning boxing's great for any sport.