without replacing the headers with proper Unequal Length Headers you CAN NOT get a proper boxer note from an NA subaru. (with the excepton of some very early models)
the boxer note comes from the timing of the exhaust pulses and when they meet together in the headers.
the pic below shows the modified turbo headers TOP
and the stock NA headers BOTTOM
one of the tassie guys put them on his 06 forester (requires some modification)
and how sweet does it sound for a 2.5 NA
http://youtu.be/V-8PEVIZenUthe actual note will then be determined by pipe size. 2" is plenty big enough for an NA
and mufflers will determine how loud it is. a nice straight thru muffler (magnaflow, redback, Supercat) will be louder than something like an STi muffler or a "turbo-flow" style muffler (Genie Turbo muffler is a great option for flow and a lower note)