Yeah, even when I run through them as fast as I can, it is rare that they all hit at once because each cannon has to turn and they're all at different distances. And yeah, I think it's the fourth one that kills the extractors, by six I don't remember seeing logistics structures at all, around 10 upgraded starbases don't have shields and are hanging out all by themselves.
The strategy I'm working on here is to mass fire them deep into the enemy territory and chase it with the main fleet. Blow the planet to hell, while a lone canon is opening a phase gate on the opposite side of the enemy fleet. Rinse and repeat. The strategy relies heavily on not losing ships to planetary defenses, though, but I can run through the ice/volcanic planets faster than the enemy can reclaim them. I want to be able to do that on terran/desert planets, too.