The birds that I linked to are not Cornish Cross. They are broiler breeds with medium growth rates, rather than the very slow that you are proposing with your cross. Jersey Giants are not nearly full grown at 20 weeks is what I am getting at, but I guess if you are at 6# by that time then it about the same as the broiler birds with just another couple of months of feed and care invested. My pullet is almost 6 months old and I estimate she is at 7-8 pounds but I will try to weigh her this evening. If my guess is right, then she is only 3/4 grown at 6 months.
ETA I personally do not raise any meat birds. I only butcher extra cockerels that I cannot sell. I am actually trying to eat less and less meat but my husband makes it hard. Store bought meat is cheaper but I do feel better eating the birds that had a good healthy life. If I was going to raise meat birds, I would definitely choose one of the "freedom ranger" type birds over cornish X though. Like you, the whole Cornish X thing makes my stomach turn, though their short lifespan is a blessing when it comes to the factory farms and their living conditions.
