I've read of some moderate "success" with crossing in some Dark Cornish, and one of our frequent posters has some big offspring with an Easter Egger, of all things (a bird that's already more genetic lottery than not). But "success" in this case means a meaty DP bird, not a meat bird like the CX.
As the other posters have FAR more informatively said - nothing competes. And yes, I used a CX (hen) in my culling project. While I was able to successfully raise a male CX to maturity via a combination of methods, the fertility rate was VERY low - easier to get some desired genetics out of the hen.
Results of my very small scale experiment? Half the child birds showed anything like Ranger-rates of growth, much less CX. Less than half of their offspring seem to be significantly larger, faster growing than their peers. Getting the "white" genes out of the mix (which i don't desire) is proving difficult.
That said, remember my offhand comment about the genetic lottery? That's my CULLING, not Breeding, project. I have a lot of genes to work with but not a lot of birds. If I started with 50 birds of the same two species breeds, and carefully bred only the best of the offspring to the best of their siblings, or backbred into the best parent stock, I could certainly make faster progress on a single trait.
officially, we raise pekin ducks for meat. Or would, if we had better incubation rates.
/edited to fix my stupid. Fingers got far ahead of my brain.