Sounds like hybrid vigor.I made the outcross to bring the rose comb in. When I made the outcross, not only did I bring in the rose comb, but the back cross pullets lay bluer eggs, the production is better than my originals, and they lay a couple weeks sooner than my pure birds. So far other than color, nothing bad has happened breeding wise from the outcross.
So far, I know of a couple other breeders that have done the crossing and had similar results in the offspring.