Can anyone tell me if the Fiesta ST is on the same chassis as another vehicle in the states? maybe the focus?. Help would be apreciated.

As far as I know, there are no North American cars that share the Fiesta Platform, although the fiesta is available in South America (Probably not the ST).

Is there a more specific query you have that people may be able to help you with? (i.e the suspension upgrade you mention in the topic title)

I have inquiries to make larger diameter sway bars for the ST but cannot confirm the design. One site said it was based on the 1981 fiesta made here in the states but I cannot confirm that either.

No, i'm 100% sure that the ST isn't based on that car.