Yeah, the exhaust is about as good as it can be its just not made from a high grade stainless like aftermarket stuff. Just get a high performance panel filter in the air box and the air deflector. A remap will make it more driveable, you won't get serious hp gains from it so it's worthwhile just not a high priority IMHO on st170's

ARB is the anti roll bar. You can get uprated ones, iirc whiteline used to do an adjustable one.
Replace as many bushes as you can with poly bushes. A slight drop on good suspension that can be set up properly. Good tyres, lightweight alloys.

Uprate the brakes with drilled/grooved discs and fast road pads, proper braided lines.

Going fast isn't always about pure power, the ££££'s you'll spend chasing extra hp could be spent elsewhere and be more rewarding.