I think if you call the strike zone per the rule book, pitchers will throw to the zone. If you give more in any direction, pitchers will take advantage and batters will have to adjust. If you think expanding the zone reduces walks, I believe you are underestimating the savvy and capabilities of high school competition.
I do not disagree with the notion that one umpire doing his very best to follow the rule book will call some pitches differently from another umpire doing the same. There are of course going to be differences in judgment from one human being to another. What I object to is when the guy sets out NOT to follow the rule book. If that's your goal, you'll be sure to meet it.
The umpire is empowered to use his judgment in determining whether a pitch is in the zone or not. He is not empowered to change the zone and thereby intentionally calls balls strikes or vice versa.
I don't really see any difference between strike zone and the foul lines. I find it interesting that so many umps think the one area needs expanding but would never consider expanding the other.
The deal is, if you think the strike zone needs expanding, make your pitch to the rules committee. They decide these things, not the HPU.