I think every ZL1 owner here including myself knows that they could own a Corvette for the same money, but in truth they are two totally different cars marketed to a specific buyer.
I have driven the SS and was very impressed until I test drove a ZL1. After that I couldn't settle for anything less. All the time I was saving my pennies I never once considered the Corvette.
If you look at the stats for the ZL1 in a recent road course test I believe it placed 3rd overall against some of the most expensive and best true sports cars money can buy (The 1LE did very, very well also). Those types of articles help people like myself become more willing to fork over the extra 20k or whatever the price difference is. I just wanted the car no matter what and walked right by that beautiful, shiny, 2013 Grand Sport on the showroom floor to buy it.
And as others have said, seat of the pants can be deceiving. Line up a stock SS vs a ZL1 and all you will see is tail lights which will become more distant as the awesome top end power of the ZL1 takes command, where the SS runs out of steam at 155 mph. I drove it wide open in Utah at the Salt flats and had it up to 171 with plenty of power on tap seemingly.
This is the video that sold me.