That OS Giken is total trash you should give it to me for my open diff
Nice car, great starting point to have almost all the suspension parts sorted out. That's equipped to handle big single type power. Downpipes are the next logical point, if your power goals are 400 wheel you can basically stop there. But why stop at 400? You have a suspension setup that will put down far more than that, we all know the motor doesn't even sneeze south of 600+ whp.
I can tell you that catless downpipes + resonated catless mids + Meisterschaft exhaust sounds pretty sweet. At least in my biased opinion. Can get you some sound clips if you'd like, though I need to figure out some means of hearing it under load. Totally different than just revving it in neutral, of course.
What are your goals for fuel? Pump gas only? E85 mixes? Meth?