I was browsing at Ideal Hatchery's web site this evening and noticed they have some meat birds that I have never heard of before. They are called black broilers and red broilers. This is what it says about them:
"When broiler rations are fed, producers can expect to have six pound broilers in seven weeks or less. Since black broilers are more active than Cornish Rocks and grow slightly slower, they rarely experience leg weakness or Ascite, which is fluid in the body cavity."
You might want to check them out...sounds like that might be more what you are looking for. If they really do flesh out that quickly, I don't see why they couldn't compete with the cornish x's. Maybe someone on here knows more about them or can speak from experience though. Good luck with your search!