The ability does not need to be stronger. A one click counter to Lefh costing 30 munition would simply be OP, especially with little counter play.
Firstly a few things you have got wrong.
There is counterplay due to how the guns operate, they will target the unit barraging closest. So by using a dummy mortar you can shield any howitzer behind it.
One howitzer 30muni barrage even in range will not reliably kill or decrew a lefh. You need two howitzers for good odds to decrew so 60muni + not bring able to build units. Then the axis can return and repair/recrew.
Axis howitzers can already counter barrage at vet1, it's not op
Brit counter battery can already target howitzers on most 1vs1 maps and even small 2vs2 maps. It's not OP
Yet in 4vs4 it can't hit leigs in the middle of some maps like steppes... It needs a range buff