Best performance mod is suspension, brakes, wheels and tires. The word 'performance' is not equivocal to speed.
For speed, exhaust and header are the best places to start; the exhaust system will always be needing an upgrade whether your ride stays naturally-aspirated (NA), or forced induction: supercharged or turbo charged (FI). A port-matched and polished intake manifold would be an expensive but good option for NA, as well as lighter weight pulleys. Other engine mods (like cams) are limited by a lack of tuning (i.e., electronic control; fuel, timing, ignition, and cam angle adjustments for optimum engine performance.)
It's been debated here whether an intake provides gains over a throttle body spacer or plenum for more air volume before compression. Evidence points that a spacer may be best over an intake.
US$.02-- worth what you paid for.