Finally processed an American Bresse rooster cockerel today, along with a couple of other mixed-breed cockerels. I could tell very early on that the Bresse was much meatier than the dual-purpose mixes (one was Black Australorp/New Hampshire Red, the other was Gold Laced Wyandotte/NHR), but the post-processing weighing confirmed it: the AB cockerel came in at twice the weight of the smaller of the two cockerels (the BA/NHR).

All were about 5 1/2 months old - I think the AB cockerels could realistically be processed at 4 months and still provide a good-sized carcass.
The AB is resting in my fridge and in a few days, I'll be making my favorite lemon roasted chicken recipe. Will be back for an update after I have a chance to try the meat; so far, though, I'm very impressed. Already planning to have an AB breeding pen, along with a FBCM breeding pen. Wonder if an ABxFBCM would be a good meat bird and/or layer...might have to find out!