see if you can find a copy of hyperrev 75 (forester) - it doesn't cover our car, but does have plenty of interesting info and leads on aftermarket suppliers.
as you've probably realised, a lot of the WRX parts can be used.
check out club f-star (japanese fanboy site) - check out the member cars. There's a few sg9 owners on there - with mods listed, so that's another good place for hints.
Anyway, be very careful with the modifying! My FD is a good example - I 'invested' bulk $$$ in that because I couldn't help/stop myself. I didn't intend to sell it though, but I also didn't intend to stay in Japan for as long as I have (going into year #7) - so it's gone, I'm still here, money 'invested' is someone else's gift, and I'm starting all over again.
I'm limiting modifying to a cat-back for the burble, and a few STi upgrades off yahoo, when I find them.
If the car is anywhere near as nimble as my old st/b (255ps stock + exhaust @ 1380kg) I'll be happy - that thing rocked.