Coat type mutations can make growth rates crappy, but if you are using standard coated rats then the difference in growth rats is mainly due to the genetic quality of the lineage of rodents, and their diet. There is no need to outcross rats if you have a good line going on. This undoes all your selection efforts and is a waste of time. Keep selecting the babies from the best rats you have, as future breeders. Take into account if they are aggressive, have good sized litters, stay healthy, raise their babies well and babies have good growth rate. If any of these aspects are not desirable in the rat in question, don't use any of its babies as breeders, and replace that rat with the offspring of another really good rat.
As for food, a rat raised on a staple of rodent pellets/cubes,lab blocks will do far better, grow better and be more efficient at raising babies, than a rat raised on a pet shop mix, or horse feed and dog feed etc. The food is made specifically for rats to keep them in peak condition. If your rats don't do well unless you supplement them with dog food and caviar (haha), then you have genetically crap rats and they are probably wasting your resources. Good brands include Cummins (McManus rodent food), and Lauke Mills rodent food.
Pet shop mixes are the equivalent of feeding wood shavings and mars bars. They don't eat any of the lucerne (green stuff), and the other bits are not a healthy diet so your rat won't be in peak condition for getting you the most out of the time and money you put in.
