The only time a chicken loses feathers due to nutritional lack is if they are seriously malnourished, so increasing proteins when birds start to lose feathers is the least of the worries if they are losing feathers simply because of being malnourished...at that point they should have an increase of everything and not just protein.
Increasing proteins can be warranted for short durations to recondition after illness, to get ready for show, for high production animals...but normally the proteins percentages in milled feeds have been formulated for commercial layers, the highest production birds coming down the pike. If they can produce as much as they do on 16-18%, any chicken can do the same.
That is why I hardly ever increase proteins fed, but I do lower them on occasion. During high production or during molt, I'll feed straight layer ration, which is the highest protein levels I normally ever feed. My birds get a lot of proteins out on forage, so I am confident that the supplement of the grain feeds is just that..a supplement.
The only time I've ever intentionally increased protein in formulated feeds was when I got that sick flock back and I didn't do it right away...the last thing a sick bird needs is to get their bowels stripped out on high protein feeds. After their initial recovery and to put fat on them before winter I added just a dab of calf manna to their feed. That was the only time in 37 years I've increased proteins past the percentages normally found in the layer rations.
High protein feed can be just as detrimental as giving too low of proteins and has just as much potential for causing illness, organ damage and eventually death.