I think we're over playing the OP factor of canister shot because it easily has the potential to be horribly over-powered, and we don't want to see that.
Canister shot could be a good addition if properly balanced. You shouldn't be able to wipe out entire companies with a single shot, or any such nonsense, as I can easily envision. It should be just enough to make a few men zig-zag charging cannons a dangerous prospect, and add some element of danger to charging a gun crew. However, a company charging, even in a frontal assault, should be able to decimate a lone cannon crew - though, of course, with a conservative price paid in casualties. It should be a last resort, close-range munition requiring proper timing and fore-sight. The main role of cannons should remain in M&M, as they were in reality, as a means of long-range bombardment that, while not as effective as a close-range musket volley, can still wear down, demoralize, or deter an enemy.