Cornish X and the Cornish breed are usually considered double breasted. I have read that the naked necks are so should have a similar breast to the Cornish.
No they are not "double breasted" unless from a broiler strain.
I've read reports comparing on naked neck vs normal broilers raised in hot climates that seemed to say the NN ones had more meat yield than their non NN broiler relatives. So it might give them some more meat than non NN but it won;t be a double breasted NN vs normal breasted non-NN.
The other thing to be aware of, NN are very very often mixed with other breeds so it is very much "your results may vary" just from this reason alone.
I have a lil turken baby that is about 5 days old now. It's a Mille Fluer D'uccle turken cross, but is all yellow fluff with some white (I think) feather showing up. How is that possible? Any idea if it'll change.