Maybe lowering the cannon kill bonus is fine, but giving it to the other team is a very bad idea i guess.
It would call for new strategies like "dont build units and let them kill our cannon, so we can strike back with a complete counter army nearly as the same value as their, because they was getting the mid bonus for a while, but then the cannon bonus equalized it and we know exactly what are their unit composition too" for exapmle.
Btw there is a mechanism in the game, i dont know it has a name or not, so i will call it "The Power Of Survivals" from now.
The Power Of Survivals is when a player is able to handle an opposite player's army and he/she will has remained survivals after that epic battle. These survivals will engage the next opposite player's forces and will weaken them, so the next ally player will has an easier job and he/she will has survivals too, and so on.
Im telling this because if the enemy is able to get to your PF (or cannon) and the PF is able to defend itself, and the defender team will have survivals, then they will can do their thing in the mentioned way and they will be "pushing like a hell" like Jarno Trulli said.
So there is always a chance for a u-turn, unless the enemy team has a much stronger army and the game is already over.
I hope you get what i wanted to mean.