I can't see any middle ground where a 300-400mp howitzer could ever be a good idea. Where the hell is that sweet spot?
Make the barrage make a loud noise when you activate it, make it lengthy and give it a wide barrage radius. The opponent has time to react, but their team weapons are screwed if they stay put.
Unless you want to severely buff the Pack Howitzer and add a fuel cost, a half size barrage is going to be awful, one way or another.
I think making it 45 FU and balancing it accordingly is a good route to take, personally.