Idano,
I do use X-Dies, have so since their introduction, and what you say is correct. When I stated +1 for what ranger335v says, I was referring to his statement about adjusting FL dies according to instructions. Doing so does, or can cause excessive headspace. You are absolutely correct that the majority of case stretching is caused by the extraction of the decapping/neck sizing mandrel. However, incorrect headspace from improperly adjusted FL dies can contribute to stretching.