Wow, interesting question. I assume when talking about FCR you mean this with the intent of raising them for meat production. My gut instinct would be they will perform like any other purebred, with an FCR around 3.0. Dark Cornish were used exclusively for meat production 100 years ago, so perhaps they had improved the breed back then. But I doubt any of that will still be present in today's birds, which surely have been outcrossed to improve their laying ability.
My guess is they would be on par with any dual purpose bird, with the caveat that dual-purpose birds aren't particularly meaty (I think dual purpose means "better than a Leghorn" in all practical senses).