The only time hybrid vigor will fall off is when the birds become more related. Don't know the long haired hippy word for it
Dave my Slow Grow CX were also excellent layers.
Tomorrow I will take pictures of two barred rock cockrels. Both are the same age same source. The difference is one has received 24% protein the other 18% the diffrence in size and feathering is nothing short of amazing. Point being that heritage breeds also benefit from quality feed.