The negative timing at launch is torque management taking its toll. Not only does it reduce timing during shifts, it also retards it during a hard start from a standing position. You should have fun with LS1 edit. It will take some time, lots of reading, but it will be worth the effort.

 

Funny, my 02 5.3L could spin the stock generals on the first day. I could never run at the track with the stockers either. I can't even run BFG DR's anymore without spinning. MT et streets are the only thing I can hook with.