One of my Ds's moved to Tiffin Sixth Form from a co-ed, non-selective school - my experience is recent.
To be honest it wasn't at all what he expected - they take in very high achieving boys from elsewhere but not all of the boys who have been there from Y7 are bright so he was easily able to hold his own academically.
He has not found it particularly competitive - most boys do want to do well but not in a nasty, putting-everyone-else-down sort of way. I'm not aware of any bullying - not every boy is a paragon, of course, and there is some bad behaviour just as at any other school.
The school is very Maths/Science heavy - the vast majority of the year group (about 75%) took A-level Maths and many do the sciences. Classes for humanities/languages etc tend to be smaller but Latin is popular.
Maths teaching overall is very good although there is one teacher every boy despises as that teacher is useless. Physics teaching is generally weaker than the other sciences. I can't comment on any of the humanities subjects I'm afraid.
The current headteacher is highly respected but she is leaving at the end of term. She is being replaced by the Deputy Head which will be good for continuity.
At some point they will (hopefully) demolish the horrible shed-like thing which serves as the dining hall - the food is rubbish, though, so some boys like to wander off to the joys of Kingston's cafes.
If your ds will be travelling far it is worth knowing that in U6th they can come into school at 10.10am rather than 8.30am if they have no earlier lessons.
Hope this helps.