Yeap just get an extractor that is designed decently that can scavenge exhaust pulses from other runner will do. As for midbox depends on how you want the sound to be like lol. Some said placing it further back , close to trunk area will have better power. No idea bout that.Yeah, extractor wasn't very expensive. A good idea to do it. How about that midbox/bullet thing? Ignore or do it?
Maybe DOHC does make some difference. Izso once written a post about air filter before over here. He did said air filter din't make much difference for his 4G15.
Here it is.
Somehow it feels more torquey without air filter for my car.
Open pod on my 4G91 is sluggish during afternoon due to heat soak, when the weather is cold though the response is very nice hahaha. As for current set up, im using drop in with stock airbox.