Hi There.
Most birds this age will be molting at some point this season, where they stop laying eggs and lose old feathers to regrow new ones. It usually happens around now when day light is diminished because that is what triggers the laying hormone. They usually return to lay in the spring.
The only main differences between "layer" and "meat bird" is two... protein and calcium levels.
Non laying birds do not need the added calcium. Usually around 4% total. Layer feed is often around 16% protein which is really about the minimum to keep a light breed laying bird in decent condition. Dual purpose birds will do better with higher protein levels.
Meat bird feeds are gonna usually be around 20-22% protein and 1% ish calcium.
I personally never feed layer and always use a 20% protein flock raiser. I offer oyster shell on the side free choice for the laying birds. Many people also switch to the "higher" (than layer) protein feeds when their bird are molting because feathers are made from 90% protein and it's amino acids.
Protein is required to build muscle and to grow. But to me... I'm not sure it will impact your birds weight because they will eat to meet their ENERGY need... meaning caloric. Energy comes from 3 sources fat, protein, and carbohydrates which includes fiber. Chicken feeds never contain caloric value information that I have seen.... And I'm not sure what impact extra calcium content will have on intake in that regards... since it should still provide the same calorie content but just be a little more bulky??
Really.. Butcher them when you are ready. They will only reach a certain size and shape according to their genetics. If you will be plucking... preferably when they don't have a bunch of pin feathers.
You may be able to "fatten" them up... by keeping them in a more confined space to limit movement... but you don't really wanna decrease muscle size either. Adding artificial light and access to feed during that time could help... but that might also bring laying back on. They can continue to lay... well even, for many years. ALL birds go through molt no matter their breed. But it's true sex links are known to have greater reproductive cancer type issues. So you may still have the right idea!
Now, my opinion... You are what you eat.
Ride this train for a minute...

22% protein was shown to give the best hatch rates... which to me says more nutrients in my eggs.. which I would be consuming... and there by... *MAYBE* even more nutrients in my chicken?? Many non medicated "starter" feeds have 22% protein.
Some "meat bird" feeds might have other vitamin or mineral added to reduce bruising... as I have read could be a tactic when transporting (water fowl) to processing. And some laying feeds have things added to enhance yolk color...
Seriously though... it cost me about $2.50/ per month per bird to keep. They will be approaching their maximum mature size at the age you state. If you need more fat... add some butter when you cook. Reduce your feed cost and energy to keep them as SOON as you are ready!
Remember... rest your bird. Not less than 3 days to let some process take place for a less tough bird. Then because of age they MUST be cooked slow and low or they will be tough. If you are used to market birds they will still be chewier... but worthy and delicious. We make enchiladas, tacos, shredded chicken blah blah..
That being said... there are other things that effect egg production as well... such as predator visits and parasites... but I do agree that it would be fairly expected... again at the age AND season.
Decide you method. For the hen(s) you will be processing, we put them in a kennel once they have gone to roost for the night, where they will be held so I don't have to do any chasing. It also helps to empty the crops and the intestines a little. With no experience and no real helpers... I can only do one or two at a time. Yesterday I had a helper with experience... but we are both a little slow, and it took us 4 hours to do 9 birds. I like doing less.. because I chill them in my fridge instead of the ice chest and just simply rinse them. They never see a bloody water bath really.
We also have a meat grinder... and use that as well. Seasoned correctly it cooks up yummy. It was a good way when we first tried skinning instead of plucking... due to the horror stories of the stench that would come.. okay, rambling now.
My final word... do it when it is convenient for you. Sooner is better than later... give ya time to eat them up before ya move.
Good luck!
