I know the name would have you think that but Cornish X are definitly NOT a cross of a Cornish over White Rocks, I don't even know why they are even still called Cornish X, they don't look anything like Cornish. They are highly developed hybrids resulting from a 4 way cross of selectively bred strains. Strain A and Strain B are bred together to produce a large meaty male line, and Strain C and D are crossed to produced a female line carrying recessive dwarfism ( so they are smaller and don't cost as much to feed) and with more emphasis on production and feed efficiency than the male line, and the the resulting birds are the "Cornish Cross" that are produced for meat.
A simple cross of a dark cornish roo on barred rocks, may will result in a bigger breasted bird if you use a good quality cornish roo, but other than that, they will be about the weight of a typical cornish or barred rock and will be about the same as far as grow rate, they aren't going to be near as big or grow near as fast as commercial CX.
