It depends on what spec your engine is. The exhaust size should be matched to your engine - ie: if you've modified it with bigger/better carb, extractors, ported head, etc... then having a bigger exhaust diameter will help your engine breath better. & will increase your performance.

But having an oversized exhaust will kill your back pressure & defeats the whole purpose of trying to get your engine to breath better. I stand to be corrected, but as i understand it, back pressure creates a type of vacuum which sucks out the exhaust gases, making more room for your intake mixture.

My uncle used to cut his exhaust just after the centre silencer & fit a larger pipe through to the back (he did this on his '85 sunny, as well as his other old cars - i always used to ask him how he got his cars to sound so nice.) Im not sure if it provided any performance benefit or if it was purely for the sound it produced.