They are not meat birds but they taste just like chicken. I butcher spares at about 15-16 weeks. Well after meaties. Not tones of meat but worth the effort.
Of all the heavy breeds you can get, I find the BSL to actually be the fastest growing. I make them myself using my own RIR's and Barred Rocks. They often become the dominant cockrels because they do grow so much more quickly than 'purebreeds' (remember there are no breeds of chicken).
I would say at the 15-16 week mark you would get heavier birds than anything, excluding broilers or possible Dark Cornish. In the same breath, the carcass conformation will still look 'funny' to you. The brest will be thin with a protuding keel.
But BSL's are certainly a better route to go than any other purebred, IMO.