I disagree with that logic.
COPO stands for Central Office Production Order. That means you could order the vehicles directly from Chevrolet, with whatever options you want(well, whatever works, anyway).
Some of Don Yenko's Camaros were COPOs.
Now, it IS stock, as it IS out of the factory. It's just a faster stock version.
Basically, you could order a COPO Camaro with a straight six or a 427.
I will, however, give you this; Muscle cars DO lack on top speed. But, if their gear ratios were longer, then they could be going faster.