The difference is 20 advertised horsepower. It's a difference that you can feel but it's not significant. But IMO there is a lot of other differences.
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. Ticker front anti-sway bar
15. Modified steering rack to reduce the turning radius by 2 feet
16. New front brake pads
17. HUD was standard