Cold air intakes(CAIs) sound better but don't improve power.
Headers like re-gearing can change the power band, it does not improve power just moves it from mid RPMs to Low RPMs or High RPMs.
You can adjust spark plug gap, 4.0l should be .054
.060 would give you better low RPM performance
.048 would give you better high RPM performance
Basically the more fuel you can burn efficiently per cylinder the more horsepower you can get.
To burn more fuel efficiently you need more air, that's what more aggressive Cams do, and/or turbo/super chargers do, the more air you can get into each cylinder the more fuel can be added so the more horsepower you can get.
Power to weight also means less weight = more power, lol
The lighter the vehicle the better the performance.