I'm not one to usually replace a bunch of stuff, but I'll bite.
Manufacturers make a product that has to conform to a certain set of rules forced on them by different governments around the world. As such, compromises are made at the expense of performance in some cases.
As an individual owner I am free to toss that careful engineering out the window if I don't want to live with such compromise and create a bike than may run better but at the expense of a little extra noise or pollution.
What makes me snicker (mmmm, Snicker's) are people who think the machine that comes out of the factory is as good as it can ever be.
Go hang a salami I'm a lasagna hog