This sounds great, but with the recent rule change i like the current mortar.
It doesn't have S4 but it does have AP and a large template.
I've played games with them now, and must say, i love them ^^.
large template makes you able to almost always hit something, and you can hit more then 1 ting. Thus making up for the -1 str.
say you hit 20 orcs. that would make 5's so wound and make it 7 kills.
Now a stone trower would get 12? to hit on 4's that makes it 6 kills.
Now if we scatter more then 4" the mortar still hits, but the stone trower doesn't.
I vote, keep mortars as they are.