Mondeo MkI/MkII V6 (or Scorpio Cosworth) calipers and 278mm disks are popular mod for Ford Sierra (I have set of this for my Roadster).
You need 11mm spacers between caliper and hub, 10mm longer bolts - and minimum 15" wheels.
But - if you need more breaking power - you can use Mondeo MkIII calipers and Focus ST170 300mm brake disks. Maybe they look different, but they fit.
You need 15,5mm spacers between caliper and hub, 15mm longer bolts - and 16" wheels
Info from my friend who is real "Sierramaniac"