After starting cold engine in the morning and idling for a few minutes white oil smoke comes out of the exhaust incessantly and only stops when I drive away.
I had the turbine modified to thrust 360 a couple of thousand miles ago but since the modification I drove the car with a faulty waistgate (it did not open and my barometer at idle did not show any underpressure).
Can the faulty waistgate have damaged the turbine?

How many kilometers / miles on the clock?

Also mine Coop has smoke when start, it is probably due to valve-rails (I don't know how to say in english, in Italy we call it "guidavalvole").
When they are cold, they become smaller, and oil can goes through them inside the engine.

No problem, you have only to warm up the engine before go away, and check oil level!

White smoke at start up = condensation ( normal)
White smoke in large quantities even after driving for a while= burnt headgasket (not normal)
Blue smoke= burning oil ( not normal)
Black smoke= unburnt fuel ( not normal)

as a general rule white smoke is water in combustion, head gasket, condensation, some water cooled egrs can leak, water cooled intercoolers in the same way. with head gasket you will often find a cream/custard like scum on oil cap, a tiny amount of this scum may be normal(again condensation). if white smoke is only present on startup or when the engine is cold its probably only condensation (i think high humidity can also cause this effect even when engine is warm),
grey smoke is over fueling. blue is oil,

I solved the problem by changing the turbo with a second hand one in perfect condition. The turbine repair shops do a lousy job in Greece.

Old thread I know, but if I remember correctly, white smoke at idle and on engine overrun (i.e. no load) is a sign of turbo bearing failure.

When I had this on my Coupe I remember stopping at traffic lights, then being passed by a large cloud of my own exhaust smoke!