3'' exhaust open up the downpipe and catback to allow more velocity flo of the exhaust, this can help with tubo spool, help the onset of boost, it just makes alot of power, verses 2.5 which is more of a restriction, the 2.5in does the same as a 3'' up until a certain point then it acts as a choke for boost and power.
some people do go to 3.5-4 ie: skylines and supras, making really big powerr where 3'' just wont cut it at 3bar of boost
some people do go to 3.5-4 ie: skylines and supras, making really big powerr where 3'' just wont cut it at 3bar of boost