I have not had coffeee yet, I apologize if this is unintelligible. I stayed up WAY WAY too late last night, after a long day of work on the barn (muscle cramps).
The science says that a number of things - disease resistance, egg size, egg frequency, rate of weight gain, and (particularly when fed to young chicks) even feed efficiency. But the improvements are tiny (measurable, but ghenerally not noticeable, typically 1-3%) when going from 16% cp to 20% CP. They are even smaller when going from 20% CP to near 30% CP. So its just not cost effective for most birds to feed the higher CP feed in most cases. Additionally, there is concern with waist protein contributing to excess N in their poops, usually as ammonia.
and if you have a mxed flock with ducks or geese, those very high protein levels can contribute to to rapid development, leading to angel wing.
Since the Met and Lys levels are so similar, for a typical flock, assuming they are at the same price point, I'd feed the Kalmbach though the Pol's is technically superior. If I were raising meaties or game birds or turkeys? The Pols'. If I could seperate flocks? I would do exactly as I'm doing right now - I'd feed my hatchlings on the Pol's (assuming similar price points), until about 8-12 weeks, then feed the adults the Kalmback.
If I had lots of hatchling birds and lots of space from a big spring order? I'd split the order in half, raise half on Kalmback, half on Pol's to 8 weeks, then compare cost and condition and report back to BYC your findings. I know the science (at least a little), still building the experience.