Lower around 2 inch on 18s would look phat.
And considering its the 1.4 litre engine it will slow you down considerably. i have the 1.6L mk6 and im still unsure if i should get 16 or 17 inch rims. will be lightweight either way though. but yeah 18s would look phat
look in the fiesta forum here on the website for some cool pics of mk6's lowered on rims!! simhoppers car is a nice example.!!!! yiew, let us know how u go :)