Tony407 makes a lot valid points but seems to be moding the sony system. tvp90 is asking about moding the six speaker non sony system to get similar results to the sony system.
So the simple answer is most likely going to be that changing the speakers on the non-sony system one for one is not going to make a dramatic difference. Now adding an aftermarket EQ and adding sub w/amp would make a much bigger difference.
Ingot Silver SEL Sedan w/303A, winter pkg, remote start, Arctic White Interior
If in doubt, pull fuse 67 and 79.