the ST170 bottom end is pretty tough, the german touring car version of it was running 240bhp (normally aspirated) with exactly the same bottom end, but it had a much better head etc.

its weak point is the dual-mass flywheel but you can get uprated flywheel and clutch (easiest to get one for US SVT focus)

i know a few people who have got over 190bhp from ST170's, by just putting on an adjustable exhaust cam pulley and having it set up at power engineering, bigger throttle body, UDP, induction kit, chip and exhaust.

the ST170 has better stuff over a normal zetec/duratec 2.0 (mk3 mondeo's now call the normal 4-pot a duratec) such as bigger valves, variable valve timing, better inlet manifold with variable tract (sort of like "secondaries"), tubular exhaust manifold, better cams etc. but even with all this they only rung 35bhp out of it over a standard 2.0 zetec (but now we know its fairly easy to get another 25bhp on top!)

you can get a jackson racing supercharger for it which will take it over 220bhp (costs about £1500 for the full kit) or the vortech supercharger kit gives it more like 100bhp but costs alot more.

throttle body conversion would work well on it also. GSXR750 TB's fit almost perfectly onto the ST170 inlet piece minus the variable inlet section (handy bit of info for anyone who wants to do this on a normal zetec!)