http://northamptonmotorsport.com/
These are the guys for a bolt on throttle body kit for your ST170 they can supply a start up map too.
Gains are not as good as you might expect to start with, the basic kit from Northampton MS will get you around 195-198bhp. To take it further you will then need to play around with after market cams but the VVT is quite sensitive to this and setting them up can be a mare, so the best thing to do is to remove the VVT altogether if planning to run wild cams, you should see well over 200 with a decent cam. But removing the variable intake and switching for individual TB and removing the VVT is taking away from the engine 2 features that make the power really usable so you have to play a bit of catch up before getting really good figures.
The Biggest hurdle you will hit is when going the N/A route RPM is your friend, the more RPM the more power, the ST170 and normal Zetec engines run the oil pump from the nose of the crank, RPM higher than 7600rpm becomes risky and beyond that is into the unknown, the oil pumps seem to fail with no clear pattern. For a high powered N/A engine you realistically want to be revving 8000rpm and beyond. So only solution is Dry sump but that will cost you more than the TB's as it will have to be bespoke.
The DM fly wheel will not like all that high revving so a lightweight solid fly and uprated clutch would be a decent idea.

A one shot map at dreamscience may bring your ST170 up to factory power again, donw hope for much more that 8bhp