So I am coming to the conclusion that canister shot is just too good given what the battle AI likes to do in the face of cannons. I have tried this in some custom battles and on a few GC battles as well.
Two caveats, first, you have to shadow their cav, if they don't have any, no worries, if they do, you need to stick your cav on them due to cav's limited AI in battles. For me it seems cav AI goes like this,
IF there is artillery
THEN charge and attack artillery
IF there is no artillery
THEN still and do nothing till it's too late anyway
So they will rush your cannons if they aren't blocked
The other one is that you have to have the canister shot tech and at least four artillery units in your force
I do my layout like such (this would be for six artillery units)
ICCCICCCI
H IIIIIIII H
You pack the rest of your army in behind the front line and inside the columns marked by your cav units
Now any human opponent with half a brain is going to horse shoe that formation and crush your sack, but the battle AI actually forms into a V shape, and this is the best part, and closes to firing range with your first line but does not charge cannon formation, they actually target the infantry most of the time. There is no flank envelopment, they basically funnel into your cannon's overlapping field of fire.
Don't get me wrong, I don't want canister shot toned down, it does exactly what should happen when you're lobbing watermelon sized shotgun shells at densely packed and relatively unarmored humans. But the opponent either needs to push the flanks or just charge the cannons, you don't what to bring a musket to a artillery gun fight.
I would also mention to offset your general from the pack as if they are using artillery themselves you're bound to loose a few guys and they always aim for the pack in my experience.
This is on VH/VH