very exhausting subject....lol..
First off a good exhaust can give you more torque and more top end. Most people don't have a clue about firing order or tube size for torque, velocity, or scavenging. and the advantages of the H or X for pulse equalization.
The first thing you always need to remember is engine size and RPM because of a simple thing called physics you can calculate the volumetric efficiency of the engine and then you have a max flow in. Then you know spent combustion gasses going out will always be less than that amount. ( unless you supercharge or add NOS )
Any time you go larger than the volumetric efficiency you loose power as you are loosing the pulse of the exhaust to pull the charge into the next cycle.
If you go smaller you gain bottom end torque but too much you loose the top end RPM power.
Manufacturers build the exhaust for the base of the applications and make it emission friendly which also cuts on performance in most cases. They put a lot of research into the exhaust to get the performance to cost and emissions balanced out.
You can make a custom exhaust with 180° pulse cross under headers spiral collectors and tuned primaries to create the max for scavenging and mate it to a free flowing system sized to your needs but that takes time and money that most people aren't willing to spend. so we have to stick with the base mid line of performance that the afermarket manufacturers are willing to make hoping to make a profit without pricing themselves out of the market, SO choices are limited as to what you can just pick up.
As for function and reliability the factory exhaust is great but for performance going to a custom exhaust will help if matched correctly to application and engine demands.
The cast log manifolds are very restrictive but are great for being maintenance free, as opposed to headers which are great for torque and scavenging on upper rpms. but can be a maintenance nightmare warping flanges and blowing gaskets... so never cut corners on quality if you want reliability.
Sorry for rambling... I better stop or I'll get into the whole thing about stepping sizes and such to generate better pulse and help stop reverberation at lower speeds.