LOL... honestly - why did you buy the Z?
Speed in a line - maybe get a V1 / passport
Speed in a line @ the drags - get some sticky rubber
Handling - Maybe some sways to stiffen her up
Sound - Get an FI exhaust and call it a day
But if you got your car to enjoy it - get some track time and an oil cooler - it will be the first and only mod you will ever need
Ok ok ... maybe not only but it certainly should be your first