Without tuning, the gain is limited. Drop in K&N has the best reputation. Some people put in all the gadgets from the market but the car perform worst than an ori car.If buy high end CAI, you better get Emanage or something else with the same or lower price. Buy chepo CAI may lead you to bad fuel consumption or lost of pickup and dirty things are not filter properly. Lead to long term sickness. Car nowadays are intelligent. The ECU or PCM will control the AFR. Regardless how good is an air filter or system, your engine suck this much, the injectors will inject accordingly. Even you don't use CAI, you open a hole in your bonnet and put a funnel onto your throttle body, you will get about the same. Then people will tell you big throttle body lar, this lar that lar.
If you are using a normal car, keep it standard will make you happy. Want power? Save money for engine transplant or turbo kit/supercharger or etc. Spend money wisely. Those filter, exhaust and etc can forget about it. Don't you see many car mod till kaw kaw but when pickup from traffic light, can hear sound very power but the car moves like a turtle.