I'm not aware of any extensive studies on this. But, IMO, if your attendance is (on average), increasing from home game to home game, I wouldn't bother upgrading too much. (The old, "If it ain't broke, don't fix it," mentality.)
Winning is the best way to increase attendance and merchandise revenue. A good PR man is a distant second to that. For newer teams, I would only recommend a high level PR man if they have plateaued. Something in the middle (lvl 4-6), might not be bad though...