I never see my cannons do any use when targeting. Anything from python down will have its Hull destroyed before you can take out a power plant or even get close. That's why I use full cannon all the time, so I dont have to bother targeting sub systems (I use kb only, so targeting is just too much dead time)
I have read that cannons only work effectively at targeting if you are hitting from the correct side of the ship. Don't know how true that is, I'm not good enough to find out. Cannons are a shotgun, just boom