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it's much closer to Essex than California style
Well in that case just cut the springs down with a hacksaw and double the tyre pressure ;)
I've never driven a focus ST but assume it's more than handy enough for public road speeds? I could be wrong but i believe you can get away with upgrading tyres if you keep the sizing the same but once you "uprate" the shocks you're into modified car territory with your insurers. If it were me i'd probably just be looking to replace the shocks with decent OEM replacements (Bilstein B4 type thing), check/replace the bushes, put a good set of stock sixed tyres on and have the tracking done. That way it should handle as well as intended with perhaps a touch more grip.
Obviously if you want to do track or feel like your hitting the limit of the car on the road then other options are available.
Very German, now reverse that order if you want to do it California style.