I don't know the diameter of the stock tips, but the Stromung dual tips are 76mm each (3") and the single tip is 89mm (3.5"). That means the dual tip has 14.14 sq.inches of outlet area while the single tip has 9.62 sq. inches of outlet area. A single 4" outlet has 12.56 sq. inches of area. Remember the calculation is only valid for the true outlet diameter, not the shroud at the tip end, which can be considerably larger.
I am planning to buy the dual tip in stainless with the midpipe as I've heard it moderates the sound in the passenger cabin. Either case, it's a sweet sounding exhaust.
[This message has been edited by Ginseng (edited February 17, 2001).]