I'll try and answer here. My DS2 is at Win Coll he has 6 days left until he leaves.
A boy fairly obviously has pretty bright, I once read somewhere over 2/3rds are in the top 3%. But as importantly if he's going to get the most out of his time there he must be intellectually curious, enjoy lots of different things and learning just for the sake of it, because there is a daily non examined lesson covering over the time he's there a wide range of subjects, he will be expected to write a weekly essay on whatever is currently being studied. Many of the dons are also very keen to "go off piste" and not teach to the examination syllabus. It's not a place for a boy who wants to be spoon fed. Winchester's USP is that it provides a broad intellectually stimulating education, it is not about teaching a boy to pass exams. Having said this I know quite a few parents think that 1. The boys feel under considerable pressure to excell academically and 2. That the place is very serious. My DS hasn't had a problem with either of these.
With regard to getting a scholarship, in the schools annual report sent to parents, the warden or head master (can't remember which) said that increasingly there is little difference between the scholars and the commoners, many of the latter now out perform the former, my DS has scholars in all his classes. They do live in "College" their food is supposed to be very good, they live in the schools grounds, so less walking, and I think there in dormitories till the top year (I could be wrong about that), I don't think they have their own garden, they wear gowns, and they do participate in what I call meaningless ritual (which I personally loath). Many boys say they wouldn't want to live in college and some don't even sit the election because of this and others turn down a scholarship and accept an exhibition because they can then live with other commoners.
If I was you I wouldn't be endlessly sitting practice exam papers, my DS only did school exams three times a year until yr 8 no practice papers and he did very well in the entrance exam despite dire teaching from his prep. The school wants interesting curious articulate boys, and of course is known for taking the quirky and the lone wolves. They don't want over coached over prepared boys however bright they may be.
As you DS hasn't even got a place yet If I was you I'd be encouraging him to be interested in things so that when he's interviewed the HM who interviews him will see that not only is he very bright but also keen to learn, able to demonstrate genuine interests in a variety of areas and be happy to talk about them.