First of all; there is no such thing as 'pre-op trannies' or 'post-op trannies'.
'Trannies' is a derogatory abbreviation of 'transvestite'; a man who wears women's clothing. Such men neither need, nor want, the surgery you're referring to.
'trannie lovers' (sic) - those men who are fascinated with pre-op transsexual women - are not interested in post-op women at all. Their fascination is with the penis, not the person, so, yes, for those people, the 'mystique' does leave after surgery.
Transsexual people don't really have a 'different mindset' depending on whether they've had surgery or not.
Many transpeople, at the beginning of transition, regard surgery as some sort of 'holy grail'; it's the ultimate goal, and the event that will validate their existence. As time progresses, most of us realise that this is an illusion; surgery doesn't define who you are, and is actually irrelevant to anyone else 98% of the time. It's only purpose is to allow you to feel comfortable in your own body; a transsexual woman is a woman, and a transsexual man is a man, regardless, and will think and behave accordingly.