Hi and welcome to BYC We're so happy you've decided to join us
When you say COBB I'm guessing you likely got Cobb500 birds. I feed mine an 18% broiler mix from hatch to process. I have found that feeding what they can consume in 45 minutes twice per day (measured and adjusted weekly) gives me the lowest mortality rate with the largest dressed weight. At 8 weeks they dress an average of six pounds per bird and I'll generally have about 3% mortality rate. Free feeding gave me birds up to 8 pounds and a 10% mortality rate.
The pro's get a 4-5 pound bird in 8 weeks with a feed conversion rate of 2 pounds of feed per pound of dressed weight. So about 10 pounds of feed per bird. I've never been able to get better than 3.5 pounds per pound.