I picked up 0.4 in the 1/4 in a well sorted mid 11 sec car by going from TTI 3" H-pipe system with 2 chambers to 3.5" x-pipe with straight through mufflers dumping before the axle...EFI took care of the jetting for me, lol.

While it might be quicker with a larger exhaust, is it worth it if it is a true street car?

Faster, Faster until the thrill of speed overcomes the fear of death...

71 Swinger - slowly collecting dust/parts
66 Belv. II - just a streetcar
88 Mustang - turbo LS beater