What are good power upgrades for a JDM car?

A safe power progression typically starts with a proper ECU tune on a factory or near-factory setup to maximize what you have. From there, bolt-on intake and exhaust, a front-mount intercooler upgrade for turbocharged cars, and a boost controller can add 20-40% power with minimal reliability risk. Fueling upgrades (injectors, fuel pump) and then turbo upgrades follow for more power. Each step should be matched with a dyno tune.