My GT shifts out of first at 40 mph, or near it, at WOT. However, mine is Series II and yours is Series III and I haven't had a chance to check out how those work yet. 80km/h for us is near 50 mph I think. Maybe one of the other 04 guys here can chat with you about that. With my older GP, I do believe 2nd gear is the longest.

Personally, I think you should be able to destroy him. A Z24 is lighter than a GP, but GPs are torque monsters, and you've already got him beat in the numbers category by 50+ in both HP and TQ. Up top, you'll walk all over him. Once you start to pull, you'll probably be far gone and giving him a good look at your taillights. Plus, your car is newer and hasn't had as much time to tire itself out or get beat on.

Edit: I was so half-asleep when I wrote this .. Fixin' a few little stupid things.