I don't think we can put the COPO and Yenko cars in the same category when considering tailpan paint. I have seen original 69 COPO cars, I remember a silver one in particular, that had regular body color paint on the tailpan.
Does anyone have an SS car that they know the history of the car from Day 1 that it left the dealership? I would put a lot of weight behind what they think.
I am not sure if anyone would want to do this, and I am not asking anyone to do it, but if you tailpan is glossy, it would be interesting to see what it looks like without any wax on it.