I've butchered barred rock, marans, and easter egger/marans cross cockerels, generally around 12-13 weeks. All except one runt have been between 3 and 3.5 pounds by that age, after butchering. I got eggs or chicks of all these birds locally. I'm not sure where they got the birds, but you could probably do better than these numbers if you get birds from people who are actually breeding for meat. If I were you and wanted to breed my own meat birds, I would buy birds from a breeder who is breeding for meat. I hear there are some nice white Rocks for this purpose... maybe someone else can comment further.
As for the feed, I agree that the protein level looks ok, but layer feed typically contains high levels of calcium, and that will damage the kidneys of young birds or birds that aren't laying. You mention that other food is available, but only in a crumble, which they waste. You might consider a feeder like this one:
http://www.instructables.com/id/PVC-Chicken-Feeder/
In my experience, there has been *no* waste at all, using this type of feeder. I will caution you though that if you have birds with very large combs that stand straight up, those birds may not be able to get their heads in. This type of feeder works perfectly for all of my hens, but I have a marans rooster with a huge comb who can't get his head in these. So just make sure that your biggest-combed birds can get their heads in, if you do try this type of feeder.
This was my last resort before switching to pellets, which I really didn't want to do. I got them off layer, and on a higher protein. They were wasting more even in a hanging feeder. I had to trim that clear lip i added, as my orp roo has a giant comb and was scraping it, but now that its trimmed, he can fit better. So far no mess!!