Callaway's were not exclusive to the C4's, as they did produce, and continue to create the C5-based Callaway C12. The basic platform of the C5 is used, however it is completely re-skinned, beefed-up motor (heads/cam/tuning).
If one is purchased, it will remain a "Corvette" on the title, and does not list Callaway on the Title/Registration.
As for performance, I've only seen one other callaway perform at the strip, and it was a '87 Twin-turbo. Unfortunately, it was an extremely hot day and the best it did was a 14.0 @ 100 on street tires. I doubt it was running in top shape.
I have made numerous runs over the past few years, recently running my best 1/4 mile at 12.278 seconds. This was done on regular street tires, factory gears, etc - right from Callaway. Recently, my trap mph was low (all of last year)..... I pulled my O2 sensors a few weeks ago, and one was completely shot, thereby allowing the car to run excessively rich (reason for black smoke out of the exhaust)......and least I hope so......
I will be out at the track in the next few weeks, looking to run a 12.0 @ 117+ if I've solved my problems; I've been chasing it for a while....