In my opinion its all going to depend on how loud you like it or can stand it. I mean, if you can get away with it legally and can stand the noise, just run a 3" downpipe with a turn down under the car. Of course, that would be way too loud for my tastes.
![:o](./images/smilies/icon_eek.gif)
Or you could just run it right out the driver side in front of the rear tire. Again, that's pretty loud.
Personally I think a 2.5" system without any muffler is going to be too loud, for me anyway. If I wanted a good flowing exhaust but somewhat quiet I'd run a 2.5" exhaust with some kind of gasspack or cherrybomb style muffler underneath the car and a straight through Magnaflow in the back mounted like stock with a 90deg bend like stock. That will flow plenty good enough for 300hp and still be tolerable noise wise.