Sonictheory, for mild mods (mild mods= mods that does not involve disassembling of your engine). not necessary to change the piping size its a waste of money only. Change the extractor ,bullet and the mufler end of story.
change of piping size is only necessary if you are driving a turbo car with bigger turbo or higher boost. For NA, increase in Compression Ratio,PnP, changing of pistons,bigger valve, cams, intake manifold, bigger injectors, engine balancing..etc
mods that will casue a huge difference from the factory spec then you will need to change piping size.
Try to help u on what i know, correct me if I'm wrong.:_: