Well, heck, nobody wants to eat a stressed-out chicken.

It's true that the meat of stressed animals is tougher, (that's the main reason why wild game is usually tougher), and they do grow slower, which also increases the odds that the meat will be tougher, and stringier. Imagine how much happier the chickens are, on tranquilizers.
To be honest, I don't see the point of that research. I very rarely eat feathers, (the only thing less appealing than a poorly-picked chicken, is an undercooked, or bloody chicken).
If the researchers wanted to prove something, they should have checked the meat. The fact that they didn't, tells you that they couldn't find any problems with the meat.
I'm not saying that these are good practices, but imagine how much better the chickens feel about the whole situation.
Seriously, I'm familiar with how the chicken and turkey industry works - there are a bunch of huge production facilities near here, and as a member of the local Natural Resources Conservation Service, we are in charge of approving plans that cover rainwater runoff control, composting facilities, and incinerators for disposing of dead birds, etc. The chicken farmers are required to build the facilities, (to meet company specs), pay for the chicks, and all the feed that goes into them, but they have absolutely no control over the feed ingredients.
The industry giants, such as Tyson, Purdue, Pilgrim, etc., take care of all the specifying and mixing of the feed. The feed is delivered by trucks, and unloaded into large hopper tanks, and dispensed by a computer-controlled, automated feeding system. All the farmer does is to make sure that the automatic feeding and watering systems are working properly, pickup and dispose of dead birds, each day, and change the litter on schedule. IOW, it's a vertically-integrated industry, with all decisions being made at the top. The farmer's only decision is whether or not to sign that contract, which requires him to build and maintain the facilities. All other decisions are out of his hands.
Tex