The "Boxer Rumble" is as a result of the unequal length headers that are used on the turbo and necessary in order to mate them up with the turbo charger. The N/A pipes don't need to take this route and the resulting noise from the turbo exhaust is the unequal pulses, like a Harley Davidson which also has unequal length pipes. Removing the silencing just makes what you have louder but on a normally aspirated engine increasing the diameter of the exhaust pipes and removing the back pressure is usually counter productive, power wise even if the illusion
of more go seems apparent. On a turbo charged engine, getting the exhaust out as quickly as possible works better hence the 3 and 3.5" systems although the diameter of the final exit pipe has little or no bearing on anything except the tone.
, there are companies making UEL headers for the N/A but don't know about the newest engine, as suggested, the BRZ pipes might be worth checking out. If you do find a set though, the system will take a slightly
different route under the car so all the pipes might
need replacing. WRX back boxes are usually too short and ride too high thus burning the rear fender and possibly
causing egress of fumes into the car.
As also suggested below, while this may sound "mighty" around town, once you get on the freeway, 'drone' from the rear of the car could make the journey uncomfortable and tiring, the better the road surface, the more obvious it will be.