Fiesta RST have always ran hot unless you improve the cooling on them! mine used to then I improved things on mine and it has made a big difference!

From what you have said it does does sound like a blockage! I would try hosing out the coolant system with a garden hose. same for the radiator. while you are having issues make sure you are using a plastic header tank as they "give" / expand a bit under pressure so you are less likely to blow a hose off!

Refit the CORRECT thermostat if you can soon, Also have a look at feeding the turbo water-jacket by splitting the feed from the heater matrix as this is on all the time and not switched off my the thermostat! Another thing that can be done is the fitting of a small radiator after the turbo water-jacket to shed more heat, this is optional as the return should go into the normal rad anyway! Painting the radiator MAT BLACK will also help as it will make it noticeably more effective at radiating heat especially if it is a shiny alloy one.

If you can get a thermostat from ford for the RST it opens a bit lower than the normal one (one of the finis codes from ford I have is 1635905 - but please check it). Motor sport developments also sell a good one too theirs opens at 78ºC. This helps by allowing the engine to start shedding heat earlier, but still letting it get up to a good working temp quickly. Red line make something called "water wetter" which improves the efficiency of the cooling fluid.

Any questions PM me! I have a few pics to show you what is what if that helps!

Brendan

PS if you can do a compression test as this will eliminate a faulty head gasket and exhaust gasses pressurising the coolant system!