I for one believe that very high 11s are
possible with a '02 Z06, just like mid 11s are possible with a stock GenII Viper.
However, I believe that it is not reasonable to expect anyone but a skilled drag racer to be able to duplicate those times, and even then only under prime weather and track conditions.
Evan Smith ran an 11.77 1/4 mile with a stock 2000 R/T10,
And other skilled drag racers have done even better (or so I've read) but I'll be happy if I can break into the high 11s with light mods (and a great deal of practice) on stock tires. Very happy.
More likely, with skill and practice, I'll be able to see low 12s.
It never ceases to amaze me what people are comfortable in rationalizing as "stock". From pulling the spare, icing the intake (not on C5s), lowering tire pressure, and doing MANY other tweaks that are unrepresentative of how the cars would be driven on the street. Also some tracks do significant track prep essentially converting the track to a gummy equalivalent of slicks, while others do nothing.
Some racers claim to be stock with amazing track times, and then revise their claims to having a stock bottom end?!?! revealing that they deem anything short of a built race motor with blower and nitrous, "stock"!?! And many feel comfortable calling ET Streets and other DOT legal slicks as "stock".

Others don't count modded intakes, K&Ns, and cat backs in their differentiation from stock, or running race gas.
With all of the extra torque that Vipers have down low, I'd say that it is
easier to launch the Z06 to hit "average" times for someone unfamiliar with their car (mid 12s for Z06, low 12s for GenII Viper).
But when I owned a Corvette, I did run against a new driver of a stock Z06 that ran a 14.0 1/4 as well, so it's not a given to get them into the 12s, let alone 11s.
But I do believe that it's possible, if not likely stock.
As for spinning and getting 13.0 a couple times, that's part of drag racing. And it's good that you are out there running the car. It is practice that makes low 12s (and maybe) better times possible.
It's exciting to read about some people getting startlingly quick times with their "stock" cars, but it can be disappointing when you actually run the car yourself, falling short of the expert drivers.
-Dean.