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.
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.