The bigger the max size, the faster they generally grow. They'll probably be 75% of max length within the first 12 months. They slow down a bit as they get bigger though.
Feeders are bad because they can introduce disease into the aquarium, and they aren't really nutritionally complete. If you breed your own feeders you take out the disease issue, but it's best to just use them as a treat. Not a staple food.
Edit: Feeding the feeders with a food that you want the predator fish to eat is fairly common, called 'gut loading'. When the predator eats the feeder, it will also get whatever the feeder has in it's stomach at the time too. Can help boost their nutritional value a bit, but they still shouldn't be used as a primary food source.