Track your food input, duration of crop, etc. then get back to me. They simply are not economically viable unless:
1) You have unlimited time.
2) Free food to feed them.
The rate of growth and FCR's don't stack up against broilers. When you do this experiment (as I have) you will find the price per pound to be cheaper going to Whole Foods and buying organic, free ranged, day spa pampered chicken.
If you want to raise dual purpose chickens simply for the love of chickens or for the experience, by all means do so. But, the body conformation isn't correct and I'd never consider selling any to my customers, they'd never come back for another bird.