Hi Again,
the fact that the car cools ok whilst driving suggests that everything is ok in regards to flow around the coolant system. The car would overheat however if left on idle without the fans working.
With the boiling after the engine has been turned off could well be what I mentioned yesterday about superheated water making its way back to the header tank from the turbo, mine does this all the time. To give you an idea how hot this is the exhaust housing gets up to around 650-750 degrees C in normal use if you pedal really hard expect around 850 + degrees C. Where the stainless water pipe runs over at the top front of the cam cover I have had the rubber insert of the retaining clip melt on several occasions!
I think a turbo cooler is a good idea, I understand that this can make a difference of around 30 degrees C to the overall cooling system but putting it before the turbo would not work as efficiently as the water needs to be cooled after the turbo not before. A turbo cooler will not however cure an existing overheating problem, it does obviously add volume to the coolant system though.

CheeRS, Mike