Probably at this time 80% are customer cars, though this could well be low, for many years, pre-sold customer cars are 90% of total Corvette production (though obviously not during the 2007-2013 time period).
Corvette folks most always want their car configured a very specific way.
The percentage will vary over the course of the model year. As the year goes on customer orders drop off and dealer stock orders start to flow. While I don't have any hard and fast facts my guess is at no time does the percentage drop below 50/50 which is pretty much unheard of for any other US model from any manufacturer.
the industry follows a predictable pattern, most product runs are 7 years, the first year (or two) it overwhelmingly leads to mostly orders - usually above 80% then as the years go on the percent slides as there are more cars on the ground that have your specs and demand is reduced as the years accumulate in the run....