My Theory
Well, i have noticed the current Toyota vehicles has a special exhaust setup for most of their AT cars.
Take example Camry 2.4
What they do is the piping dia. after the header is large (eg.3.5") then the piping dia. after the CAT is smaller (eg. 3") and the piping dia. after the center bullet is even smaller(eg. 2") straight to the rear muffler.
The theory that i get from what i see is that. They made the dia. after the header large is to allow the exhaust gas to exit the combustion chamber fast and the exhaust gas at this area is the hottest and the flow speed of the gas is the fastest.
Then after the CAT the piping dia. become smaller and smaller this is because exhaust gas give up their heat to the cooler surrounding (the atmosphere) when exhaust gas losses their heat, they losses their flow speed so to ensure a minimun drop in heat and flow speed the engineers shrink the piping dia. section by section. This forces the exhaust gas to be compress on each other thus increases the exhaust heat temp and increases the flow speed. This also creates a slight vacuum inside the exhaust system that helps pull all exhaust gas out from the combustion chamber asap.
The result they get is, a faster flowing of exhaust gas will give better and more powerful combustion as very less exhaust gas is left inside the combustion chamber.
Well, this is my theory lah but don't know to what extent it is correct. Its a good discussion though.