I will be trying this as well. I am just waiting for my hens to come into lay, then will be trying:
cornish roo x white rock hen
cornish roo x barred rock hen
cornish roo x sussex hen
barred rock roo x cornish hen
sussex roo x cornish hen
Now, I will technically get "cornish crosses" out of the first three... but you are not going to get the same birds you get through the mail from McMurray. Over many, many years, they have selected birds who showed aptitude towards eating and being sedentary. They have their own 'strain', where my F1 strains will pale in comparison.
However, over the course of four+ generations, I may be able to come up with a good bird. Or it may take longer, who knows. It's a side project. All I can recommend is keep excellent records and weight he offspring often.
I've never been happy with the mortality rate on the cornish crosses through the mail, so even if I never match the weight gain & size, I'm hoping I make the money back with better foraging birds and a lower mortality rate.