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I would think that any 3" one would give the most power. RB sells a 2.5" pipe and they also use a 3" pipe in their full system. At least that is how it was back in 96 when my T2 died.
the bigger the btter up to about 3 inches or so i would think.. but then you may aswell just run straight headers if your goin any larger then that. then theres always the need for backpreassure argument