I'm one of the people who recommend higher protein feeds for our backyard flocks, as a general rule. That recommendation is NOT based on improved egg production (which is measurable, but insignificant) or larger egg size (again, measurable but insignificant) - in both cases, typically in the range of 2-3% - which may only equate to an extra 4 or 5 eggs for a third year quality layer over the course of the next 12 months.
I make that recommendation for improved condition of your birds overall, faster molting, somewhat greater resistance to pressures like disease, injury, age - because most backyard keepers think of their animals as pets, not producers, and are willing to invest a bit in hopes of achieving a longer life. The higher protein *also* helps compensate for the tendency of backyard owners to offer excessive low nutritional value and/or nutritionally imbalanced treats, and the fact that many aren't able to free range their birds successfully (ordinances, weather, pasture options, etc) in ways which benefit the bird's diet.
16% protein feed is adequate to their nutritional needs. A bird's dietary requirements tend to decline as they age. High quality, high nutrition feed is most needed in their formative months, less required in their most productive year to year and a half of adulthood, and then continues to fall off after that as egg production declines and the bird shifts fully into "maintenance" mode. Their calcium needs drop too - so depending on individual bird and breed, you can start to see signs of calcium build up in older hens, just as its seen in Roosters offered layer feed - only the build up takes considerably longer to become evident.
As with everything, there are no guarantees in life, and in this case, there are no hard numbers, either. Studies on birds beyond peak productive age are few, far between, and generally small sample size. Only you can assess whether a chance at a longer life is more valuable to you than the cost savings per bag of feed. Given replacement costs for leghorns, even factoring the time and feed investment to grow them up to adulthood, a $4 per bag differential adds up pretty quick. At $1 or $2, the question is a far closer one for most backyard owners.
