Hey guys, I asked an active veteran Croatian air force pilot about the smoke, he could not give a certain answer as he says that it really depends on a lot of parameters such as a trained eye of course, the position of the airplane, weather and meteo conditions as well as the engine regime...he said there should be no smoke at 90% power setting.
Anyway, on sunday I am going to an airshow and will watch closely what happens with the exhaust smoke ;)