I have underdrive pullies on mine, engine pick up is instant but i have an aluminium fly also.
Wouldnt bother with cams, you would have to lose the VVT to fit an aftermarker inlet cam, the likes of Piper etc reccomend removing this as they are a fricking nightmare to set up without bending valves.
Apart from an Sports CAT i have everysingle bolt on mod you can by for the ST170, last RR mine made 189bhp, was it worth it............no do i wish i had spent the money on a ST225...........yes.
Dont get me wrong the car is fun to drive but its not quick in todays standards. Yes you will show up allot of the rep mobiles but the most fun you will have is round corners.