This is actually something I'm curious to observe in my NN/Biel cross because, overall, my NNs are pretty feed efficient compared to most of my other birds. My Dorking-mixes are the only other ones I've noticed good feed efficiency in, but they're really flighty birds, obviously taking after whatever they were bred with. (I bought hatching eggs for what were supposed to be pure Dorkings. They weren't.) I actually crossed one of my Dorking-mix females with my Ameraucana-mix rooster and got a pretty adorable, very calm and friendly chick out of the mix that I will be observing closely. I'm really, really hoping it's a pullet, but with my luck that's not likely.
This is Ziggy, so far with a 102% growth rate in its first week since hatch.
My greatest struggle is finding a breed that suits my needs and has an appearance that appeals to my husband.