Regarding the importance given to interviews, I think the HM at Hampton sums it up (when giving briefings in the interview sessions) quite closely as to what many senior schools are doing. He said they take a holistic approach (meaning they are not giving marks and/or not adding the marks from exam, school report, and interview together, Rather the admission committee will sit together and go through the list (those short listed for interview I suppose) together. They will also have a rank order based on entrance exam scores. They will weed out those considered not suitable. He said he meant those not in line with the school's ethos, for example, those without dedicated non-academic interests/hobbies, etc, as the school wanted the boys to have something to fall back on when experiencing exam pressures later on in life etc...
Of course, I doubt if they could identify it in interviews as I am sure no one will say I studied everyday and have no hobbies. Sure he will also look for other hints or look for something else that he didn't want to say in public.
But what I got is that likely they will go down the list (exam scores) from the highest to the lowest, and if no alarm bell was raised, that boy will get a yes.
That's why I concur with some comments above that the school shouldn't interview too many. Hampton interviewed about 280 for 100 spaces. if it offered 1.5-2 times the spaces, then the offer-to-space ratio will be 1.5x or slightly more which I think is suitable and indeed responsible.