I wouldn't bother with the the typhoon, I would get a k&n panel filter and take the resonator box off the side of the airbox for the sound.
I also wouldn't bother with the full stainless exhaust, the st170 already has a sports cat and 4 branch manifold, and sounds great in my opinion.
A vernier pulley set to -6 degrees on the exhaust side is said to give up to 10bhp, I've done it and you can feel the difference.
People are putting the automatic air deflector above the grill as it's less restrictive then the standard piece, and for £15 you can't go wrong.
Other than that it's all about handling, poly bush everything and a decent set of tyres.