the jdm forester sti 6mt was, theoretically, good for 200mph if you could rev to 8000rpm.
320hp driving as you stated, I'd clean it up, replace the bushings and any wear parts with least resistance parts.
@Pleides I would opt for the spec b gearing but make 6th gear as over drive as I could, mpg is mpg. Even on the 86, I skip shift to 6th gear and cruise as soon as I can. Unless Im being irresponsible, then 3rd gear only.
Get hardened gears? Micro-finish any parts that add drag? Stretch goal - powder finish the outside sti pink