I might be able to help as I have DS's at KCS and St Johns so think I have a good grip on the differences between the typical "super academic" school and the other choices
KCS (like Hampton and RGS) is big (150 boys) compared to around 70 at St Johns which makes it different in many ways. Much more competition in terms of sports teams which makes it less inclusive so can be somewhat disheartening for the boy who enjoys his sport but doesn't make the cut for the A or B team. Likewise lots of competition in terms of Drama and Music. They put on lots of vey fantastic performances but you have to be really talented to actually get a part! There is a similar theme across all the activities. In one way its great because it forces the boys to find what they love and are best at, but in another way it does mean boys sometimes give up doing things that they don't think they are great at. Academically it is very tough. Lots of homework and absolutely no leniency or tolerance for not keeping up. However, as most of the boys are naturally very bright, this is just what they need to ensure that they don't coast along doing the bare minimum. Its definitely seen as "cool" to do well academically and no boy wants to be scoring too far below the median. At prize giving you are only invited if you get a prize (and those prizes are few and far between - definitely not a "reward everyone" culture). So basically it can be quite a brutal environment to be in and its not the sort of school for a boy who lacks resilience. Socially its extremely diverse - kids come from all over the place and there are lots of parents from very different backgrounds. I would say that compared to prep school it is actually much more kind/tolerant - there are so many boys it is very easy to find your "gang". Boys are also quite street smart as they are used to using public transport and meeting up with friends across london. I would say it has a cosmopolitan feel. Great school - works really well for DS1.
DS2 is at St Johns. Also a great school but for difference reasons. Much smaller and less diverse - its really is made up of fairly affluent surrey families. In terms of extracurricular activities its vey inclusive - if you want to play a sport you can generally be guaranteed a match every week. Likewise for all the other activities on offer. I really can't fault it in that respect.
Academically it's much more nurturing.....I have to disagree with Tutti Frutti in terms of the intake. Not everyone is hugely academic - and there are a fair number that struggle and they are very good at helping them. I would say that the majority of kids are above average - capable of getting those As and A*s but only if they work hard for them. There are of course a handful of very clver kids in each year but I wouldn't say that this is the norm as these children become well known for being "really really clever" . Is it "cool" to be clever....mmmmmm....secretly I think they all want to do well but no one is shouting from the rooftops about coming top in an exam as its not that sort of place.
Socially it can be quite cliquey - there are groups of children who are perceived to be popular. I think this is because of the size of the school and the fact that everyone knows everyone. Its very like it was at prep school - so if you have the right type of child then they will have a fantastic time but for the wrong child maybe not so much. Are there parties? Yes....but there are at all schools. If your child is popular at St Johns they will probably get invited to lots of parties (or gatherings) that are in the surrounding area, hosted by affluent families in beautiful houses! My DH and I joke that its a bit like being at Beverley Hills 90210!
I love both schools - the reason we chose St Johns over KCS for DS2 was because he didn't get into KCS! In retrospect it suits him much better than KCS would have (because he is big fish in a small pond) and he is very very happy. I would perhaps like him to work a bit harder, but I am hoping that his brother's exam results will spur him on!
Finally on the subject of pretests - I wouldn't worry. Just follow the advice of your headmaster. Ours told us DS2 would be borderline but he was a shoo in for Reeds/St Johns which is exactly what happened. I subsequently found out that he had scored 100% in the math (yet still failed to get into KCS) so very different levels.