I don't do pork or beef at Costco altho our Costco frequently has beef marked Prime...but there is Prime and there is Prime.
I do really prefer the New Zealand racks of lamb that I buy at Costco to those I pay almost twice for from my carriage trade butcher. The local lamb, albeti well grown, is killed later than the New Zealand lamb, and I prefer teeny, tiny lollipop chops. Plus they are tender as love and have a gentle lamb flavor. I also buy racks of lamb at Trader Joe's, the same young kill.
My husband frequently recounts our being at the Wednesday lamb market in Espelette in France's Pay Basque. Trucks of lambs hauled down from the hills, small as cats, tossed from seller to buyer like bean-bags. One week before Easter. Now that's lamb.