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Well have fun drifting up those hill climbs!
Ok I run a sort of 3" / 2.5" hybrid system on a v6. The trick is to merge the gas flows BEFORE the cat. So you need a single cat, a single front muffler and a single resi. This is actually a reasonably quiet system. Full system is Extractors into y pipe into 3" Hi perf Cat, into 2.5" muffler into 2.5" resi.
3" is fine for the hotter gases. As they cool, they condense so the smaller 2.5 works well at the reare end.