u know your engine small and require small piping to make adequate power to move the vehicle. So if u increase the diameter of the piping anywhere after the manifold, u are only changing the power band arrival. U know why? Because small engine can only pump and vent only so much air. So in order to keep the power constant, u need small diameter pipe to retain some back pressure for the next combustion cycle. Yes there are so called rally experts that say back pressure no good. But they are running turbo engines. This they never even thought about. NA engine different. Really u spend RM600-1000 on an exhaust system only to find the power changes between gears and not overall. And changing exhaust system does not give you any power increase unless u do something to the engine. Its always psychological factor

Depends on what u want. Impossible for me to say or recommend because i would only touch exhaust as the final step after mods in engine. U can see a lot of sites that do dyno of individual parts upgrade. The best exhaust only gives out 1-2HP gains. ECU gains are the highest. But then the engine type that can make best use of ECU reflash, piggyback, or replacement are turbo engines. Almost always 30-50HP gains. That also depends on other upgrades. If you just want sound, just play around the back baffle/ muffler/ resonator. Dun waste time changing the middle muffler/ bullet.

If those abang adik exhaust shop really want to market their products as power gain products, they will create a true custom exhaust and do many dyno rounds for it and market. But they just cut and paste form other manufacturers for different car models and sell it as clones and expect it to perform a lot.