I second the cost effectiveness factor, as much as I don't like promoting the CornishX.If your motivatiob is cost, i believe you will find the Cornish X win when you factor in the cost of keeping and raising out breeding stock and feeding your broiler for months longer.
Im also skeptical of taste tests by breed, but i can see bow some might prefer the texture and flavor of an older bird over a typical Cornish x.
However if you're all set up to raise them yourself for your family, it sounds fun! Those white dorkings I've seen oon here are lovely.
To the OP: when you say cost effective, are you speaking of the cost to get each bird to the table, or cost per lb of meat? Pound for pound, even if you suffer a few losses, I do suspect the commercial meaties would win out. Which is why they are raised commercially. But, I would love to see your follow up on the project as well as your stats!