The 2.5" mandrel should make a little more power then the 2.25" being as the exhaust is still resticted by the long snaking single pipe of said diameter.
The duals after that point are of little consequence really.
If using a Y-pipe design then staying single is a better choice.
SVT owners aside for the aesthetic reasons (not looking like ass with just 1 tip )


2000 SVT #674 13.47 @ 102 - All Motor! It was not broke; Yet I fixed it anyway.