Telling people to 'focus fire' on anything is just a roundabout way of saying that the ship is too strong (after all, why would you need to focus fire it if it wasn't).
The thing is, the Conq isn't as OP as it might seem. Fire damage is nearly fully reparable, so it doesn't stick like battleship AP damage. If you nerf the Conqueror's fire chance then it won't have anything to rely on - it doesn't have USN special AP mechanics or just the raw penetration of the Yamato's guns, or any speed like the french BBs to get around to be able to pen things. Even if does use the 456's its still just a worse Yamato.