The ST170 has a dual mass flywheel, a cost- effective way of replacing this would be to wait until you get trouble with this, then replace the DMF with a solid flywheel (which can be lightened and balanced) and a stronger clutch with a higher clamping force, an organic clutch, - best to get one with a sprung plate if you are fitting a solid flywheel - for more "give"

Underdrive pulleys can give surprising (good) results, but can have drawbacks like running the ancillaries too slowly (alternator waterpump etc)

On my project car (similar to the ST170) i removed the AC system, the pump alone weighed 7kgs, the AC pump pully weighed 1.2kgs this always turns with the engine even when the AC is off, its like taking 1.2 kgs off the flywheel

I planned an underdrive pully but fitted an electric waterpump instead, this completely removes the load/ weight off the engine, the electric pump works on demand, so th engine warms up quicker/ is cooled better, the pump can be set to run after the engine is stopped, if required, it worked so well i removed the fan, to further reduce weight

Together, the AC and waterpump could be worth 10Hp or more
with the AC & waterpump removed, no underdrive pully was required, so the other ancillaries ran at normal speed, so the power steering and alternator worked as normal, without problems

I managed to strip about 150kgs out of my car, this made a big, big difference, not just in acceleration, but on the brakes and cornering too - its hard to get "more power" out of many cars - like the ST170 - and Ford had to work hard to get 170Hp out of a NA 2L, leaving it NA you are liable to gain power at the top of the rev range, and its not going to be a LOT more power - as others have indicated