The Clutch, Fly wheel, slave cylinder and uprated clutch cover cost me around £600ish delivered from CFM in the states. As for fitting i did this myself and it took me a whole weekend but has not missed a beat. For a garage to do it your looking at a full days labour so could be £250+
My ST170 has only been on the rollers once and it made 189bhp with my mods, its not been on since i tweeked the cam timing a bit more. I assume it will still be around the same.
I did my Cam belt recently and the mod your reffering too is the fitting of a Vernier Cam pully to the exhaust cam, the inlet cam as standard has variable timing but the Exhaust cam is fixed, what you can do is release a bit of extra power top end by retarding the exhaust timing by up to 8 degrees, gains of 10bhp have been noted with this mod, i fitted underdrive pulleys also which half the load the assersories put on the engine and save another 2.2kg rotational mass. My ST170 is not what i would call a quick car, it just feels allot better and livlier to drive, it should have been like this from the factory, acceleration is great in 1st and 2nd due to the fly and will top out at a GPS indicated 147mph, which was on a private track of course.
ST170's are hit and miss, some are really down on power, others respond better to tuning than others but without spending thousands you will never reach 200bhp and i have spent getting on for 2K and im not even close.
Dream science no longer do a handset for the ST170 so if you want a map from them now you have to go direct to them for a one shot mapping session