It's going to depend on the breeder. Good breeders will be keeping an eye towards production as well as keeping them physically to the type they should have. Some won't. Just like anything else, it will vary. Like, I could sell you a box of chicks from my first mating of these birds in the fall/winter, and those chicks might lay gangbuster, or quite poorly, because I don't know yet myself what I have in the pens.
They're just under 6 months old, that's astounding for heritage/heavy/exhibition birds. Two of them squatted while I was out there, so I'm pretty certain it wasn't even a glitch on the older halfbreds laying an extra. Out of curiosity, I then weighed my current first and second choice for breeders, #1 is 6lbs and #2 is 5.1lbs. Not too shabby, as the Standard calls for 6lb pullets and like I said, they're not even quite 6 months old yet.