Ok, this question comes up frequently, so in a nutshell here are the major differences between 2001, and 2002-2004 Z06's. 2MuchRiceMakesMeSick for compiling it.
differences between 2001 & 2002 & up:
1. The LS6 engine standard in the Z06 increased from 385 to 405 horsepower. Torque increased from 385 to 400 lb-ft.
2. Front fender badges for Z06 models included the 405 HP designation.
3. LS6 exhaust back pressure was reduced by 16%
4. Cam shaft was redesigned resulting in increased lift and a 5% increase in air flow
5. Valve spring material composition was changed to increase seat load by 14%.
6. LS6 intake and exhaust valves were a new, lighter design.
7. The valve train weight was reduced by 0.81 pounds, which improved valve seating at high rpm.
8.Airflow over the LS6 MAF sensor was increased by removing grid work
9. MAF software was re calibrated
10. LS6 air cleaner housing had a larger intake
11. Wheels looked the same BUT they were cast spun instead of forged for a weight savings of 1.3 pounds per car
12. rear shock valving was changed
13. Improved clutch
14. Thicker front anti-sway bar
15. New front brake pads
16. HUD was standard (HUD was not avaliable in 2001 cars)
17. 2004 introduced a new fuel system which carries over into the new C6.
The technical details on 2001 vs 2002 can be found here. http://www.idavette.net/hib/02ls6/index.htm