In Commercial lines Hybrid Vigor is all but dead(in both broilers and egg laying strains), what they use is complement breeding.Hybrid vigor is a real thing.
Parent Sire Line A is selected for the most growth regardless of egg production(5.5 pounds at 5 weeks of age), Parent Dame line C is selected for better egg production(which is about 160 eggs at 60 weeks of age) but they also have exceptional growth(5.2 pounds at 5 weeks of age) when both lines are crossed the best you can hope is the Average of the parents and that is what you get in the Terminal Cornishx cross