DD sat the 11+ test for some of the schools you mentioned this year. She has been working at Greater Depth since Year 1 and goes to a very good primary school, where's she's one of the top in the class. We had a tutor since Year 4 and did a lot of practice papers. She didn't even make it through the first round.
Of course I've asked myself if it was all worth it. But DD wanted to do it as her friends were too and I think if we hadn't, we'd always have wondered what if. The extra support has definitely helped her learning and boosted her confidence, and this will benefit her at secondary school wherever she goes. I think it really depends on the child, DD1 is very resilient and wasn't upset about what happened. DD2 is another matter and I don't think the process is right for her.
I do think it has got harder and lots of families really go for it with the prep. I'm talking multiple tutors, missing school to do intensive courses and no family holidays for a year so their child can work every day. I know these children and most of them got a place. We didn't want to do this and I will always be pleased that DD had a great work / play balance, and enjoyed play dates, holidays and extracurricular activities. The work fit in around us rather than the other way around. But it's important to understand that this is what you're up against when you apply to these schools. Latymer had 3,300 registrations for 192 places this year.
Of course some children have a natural aptitude for these exams and will do well regardless of how much prep they do. These children will most likely do brilliantly at these types of school as it's the right environment for them. I was gutted when the results came through but in hindsight, I now think DD will thrive being one of the top of a different school rather than being one of the bottom of a highly selective school.
I guess what I'm saying is, if you want to go for it, then do as you will always wonder what if, if you don't. But go in with your eyes wide open and your rational head on so you don't get too caught up in the noise! Good luck.