We've been using the Callaway for quite a few weeks in our league and all members of my team have the same issue, the car absolutely bogs off the line. We continually get passed by 2 to 3 cars, even after trying various launches, such as tc1-tc5. The only one that seems to work is tc6 with 0 traction, but as you can image it takes a ton of skill to do it properly. Hopefully the dev's can chime in and possibly take a look at it and improve it a tad...
I have always used a TC of 5% with most of the GT3 class for the start then up the level for the race, the Corvette is one of the more difficult ones though, all part of the fun