Hi, we've done this with DD1, in fact we did it when she was in Y4 (nearly 9) even though our original plan was to move at 11 and we'll be doing it for DS1 too. Not sure about the other two children yet!
The biggest driver for DD1 initially was that she was unhappy with her school day. She complained constantly that the work was boring and despite having friends this issue seemed never to go away for her. From our point of view this also reflected in the lack of progress and general sense of work ethic I felt was important.
For DS1 it was less straightforward as he didn't complain as much, but the issues were similar.
In the end I felt that without some considerable input from us at home, they would not have passed the 11+. Yes lots of other children in their school did but our situation was different. First of all with 4 children and a full time job I found it difficult to sit them down every night and start tutoring. I tried this but I had to laugh at myself, with not even being English trying to explain English grammar and struggle severely not only with the pedagogy side of it but also with the meaning - I felt I had to study stuff myself before teaching it and wasn't even sure I was doing it right - I felt it was like watching a comedy movie.
Paid tutoring also implied driving them somewhere at least once a week but then doing additional work most nights was expected and 1 hour a week would not have been enough for the amount of stuff I felt wasn't being covered.
Then there was the fact that they all play musical instruments, to quite a good level and had other hobbies, so catching up on increasing amounts of work at home was stressful and demoralising and it would have meant giving some of the other activities up.
For us the move worked really well, well it has for DD1. She is taught very well and thoroughly and homework is really just reinforcing what she has learned at school so never needs any input (just checking!).
I won't have the issue of preparing for the 11+ and most importantly DD1 is very happy and never complains of being bored. I have recently gone to see her work and I was amazed at the high quality and variety of the work produced. Her science was particularly pleasing, not something that was done much at her old school and frankly I wouldn't have thought or had the time to do at home. Apparently she excels at it and has found a new passion.