No bird will match the cornish x rock for growing speed or size. The xrocks are really for meat only.
If you're looking for a meat and egg bird (dual purpose) look in to something like a Rhode Island Red, Sussex, New Hamshire Red, the heavier breeds. The RIR is probably the best egg layer...
most cornish x rocks don't live long enough to reproduce. Their rapid growth rate causes their organs to fail (the organs just cant keep up and get over worked, then fail).
As to if they would lay well, I doubt it. Cornishs' don't lay well to begin with although the rocks lay ok. But like I...