try pinkbike.com. might be a more appropriate forum
calculate your mean flow velocity, determine pipe size, if hopped up, sonic wave scavenging, build as few bends as possible and add a market tunable muffler and tips. cross scavenging and back pressure is your freind.
PS I'm running the 3" COBB exhaust on my '06 sti...easy install, good flow, and is guaranteed to wake up your neighbors at 4 a.m.
Would definetly recommend it.
Sedan or Hatch?
If your just looking for sound, my suggestion is just get a cat back exhaust.
Want it really loud? Turboxs is loud as hell and has a better reputation than Tsudo.
In my opinion Cobb makes the best sounding exhaust for subaru, but thats my opinion.
Pics of your car!