2-1/2" should be plenty big for any car on the street. It is all a function of cubic inches and max rpm really...
My 393W is running through 1-3/4" 351W-swap Fox-chassis Mustang shorty headers, then a 2-1/2" Fox Mustang off-road (no catalysts) H-pipe, back to 2-1/2" DynoMax Super Turbo mufflers, then necking down to 2-1/4" tailpipes that exit at the back bumper. Back-to-back runs at the track with the headers open and through the full exhaust yielded exact same ET/MPH. I really like those DymoMax super turbo mufflers - one of the highest-flowing mufflers out there that actually quiet the car down. Then again, I'm not revving the snot out of my motor either - shifts are at 5500rpm, going through the finish at 4600 in Third - at 106mph! That is through the full dual exhaust, too!