Plymouth Rocks are fabulous layers, Chris. My three and a half year old girls still lay several eggs a week, even at their age. When they were younger, they would match the RIRs egg for egg, too. (Wish I could have had more for you when you were here today, but later on, if you want some, I'll make a point to collect them a week ahead and you can pick them up)
the best egg layer in my opinion is the cochin, and the red sex-links, a cochin will lay about 4-6 eggs a week, same as the red sex-linkS. I have a few plymouth rocks but they are POOR LAYERS!!!!!
Chicken obsessed, that is highly unusual for BRs to be poor layers. Normally, they are some of the very best. The strain sometimes has alot to do with it, I would guess, but generally, they are about as good as a RIR.