Stick a Vernier Pully on the exhaust cam and retard the timing should see you up to 10bhp, need to be set up on a RR though
If you want a more responsive throttle then port the standard item, its helps the engine pick up quicker, failing that there is a limit to the ST170's engine, its highly tuned as standard. With simple bolt on tuning you might get close to 200bhp but dont forget ALLOT of st170's are not running 170bhp as standard, it will be morelike 155-160, i have seen them as low as 145!!
You seem to be heading in the right direction, forget superchip blufin its pants, save for the dreamscience unit. The flexi and cat is the biggest restriction in the exhaust but be warned no cat will give it a pants exuast note and even a sport cat may not make it through an MOT. These cars are especially thirsty on fuel