There are about 120 places at Tiffin. Around 3500 kids took the test when my son sat. These are extremely competitive schools. Your child needs speed, familiarity, and a high IQ.
Unless at a prep school, it is unrealistic to expect even a hugely bright child to pass without some form of enrichment (even if not tutoring) eg they need to have somehow encountered the whole Y6 maths syllabus before completing year 6.
Once in, they are with a cohort of extremely bright, fast, motivated, high IQ kids… of course: those are the kids that passed.
Re tutoring, if by some fluke, an over-tutored child got in, who isn’t actually naturally quick and able, then it’s a hideous school life ahead, struggling to keep up. Don’t just think about the “prize”, think about the school career that lies ahead. Would a very academic cohort suit your child? Some children like being the “cleverest”. Now they will be just normal in their group, sometimes still shining, sometimes not at all. Sometimes a regular school, with the full range of abilities, or a more nurturing school is better. Plus, Tiffin has now been downgraded in its Ofsted report.
In practical terms: My child practised a paper a week (50 mins type), then this increased to one a day, I think, close to exams, in the autumn. But it was all his choice, and I think that made all the difference. Plus, he went to a very slow paced primary school, with no homework, so this wasn’t on top of loads of other homework.