Well first off there's a big difference in price between the Z06, GT500, and Mustang GT. The Mustang GT is roughly $25K, the GT500 will likely be $40K, and the Z06 is $65K.
With that said, i'm sure you could pump up the horsepower even more in the GT500, and make it handle a ton better with the extra $25K you'd have if you bought it over the Z06.
Of course you'd still have a car that is roughly 800lbs more.
Car and driver said the Z06 was 3100lbs, and Ford stated the GT500 will be 3900lbs, thats a big difference in weight which should effect quarter mile times a good bit.
The old saying is that 100lbs is worth a tenth in the quarter mile. That would make the GT500 .8 slower than the vette which puts it square into the mid 12's right where i'd say it will be with a good driver.
BTW R&T magazine said the claimed 1/4 mile time was 12.9 for the GT500