csl

I don't know how to give you a perfect answer. Only can give you a few ideas. But this is more on normal car on the street. Not those heavily modified race car. Because the application is different, where we are using various level of rpm and need to consider the fuel consumption as well. But those track car or race car, they constantly on high rpm at high speed and fuel consumption is barely in the picture.

Air filter - Sure more air goes into the chamber is better. But 1 thing needed to be remember, when the piston goes down and creating a vacuum to suck air (N/A), the displacement is the one determine the volume. Using open port or even remove the air box won't let you have more air to go in. The purpose of using open port filter, besides filtering the air, it also let the air go in easier and the engine won't waste energy when the pistons pull the air. By right it should be better. But here come the problem. When it operate, the air flow will become highly fluctuate. Because there is no guide (air box). Not to mention those cheap OP filter with uneven filter material distribution. This may potentially cause those cars having MAF sensor having rough idling. That's why the car manufacture increasing the cost by adding air collector box to reduce the fluctuation and make a maximum efficiency within the budget and the need of the average potential user. Another issue is hot air in the engine bay. Base on common OP setup done by our local shops, I don't need to explain in details, you know I know.

There are plenty of theories that lies behind the scene. But lazy to type la. Also I'm not that pro also. Just make it short, go to pro shop like Millennium or N1 or etc and seek for consultancy. Just need to tell them you want a solution to suite your practical city driving. I guess they will recommend you K&N drop in. If you can afford those high end solution like ARC box, you can let them know :).

On exhaust system, you can refer to this Theory

Just one thing, because of manufacturing cost and the production volume, no car or parts come in 100% perfect as per described. That's why you need to have at lease a piggy back to dyno tune your car. Make sure the fuel is not wasted. Just like some people use huge exhaust (maybe can stuff 3/4 chicken in for BBQ) on a 1.5L N/A basically ori car. Sounds fast but only can see it arrive after finished a stick of cigar . The air/fuel mixture that leak out during valve overlapping can probably let them drive another 100km further every few months. But that is not my business of cause.