The value of valine as a limiting amino acid in young animals has been well addressed. This has been repeatedly demonstrated in the case of piglets. Similar research is certainly sparse when it comes to broilers.
In one study, I have seen L-valine being added in late-broiler diets up at about half kilogram per metric tonne with good results. Higher inclusion rates actually depressed performance, suggesting an imbalance of amino acids, and potentially an antagonistic interaction with other branch-chain amino acids (such as isoleucine and leucine).
The question is, are we missing growth performance potential by not exploring further the actual needs of modern broilers in regards to valine?