My son is at WUS and would be in your boy's year.
WUS is very gentle and closely supervised (much more so than the preps my DS's friends went to eg Sussex house , colet, Harrodian ) The pastoral care is excellent and form teachers very approachable. Communications from the school are copious.
They do totally shut at half term, this term particularly. Remember that the whole staff has had two six day weeks to invigilate and mark close to 500 7&8+ candidates' papers, then the 11+ marking, plus interviewing the short lists for all three entry points, all on top of the usual very packed curriculum. The pace is one of the joys of the school - as the intakes are selected to be fairly homogenous academically the lessons can clip along and because the boys are all v keen and able they can afford to be a bit more informal where appropriate as there is limited risk of anyone taking the p*ss. However, they don't take anything for granted eg boys who get the lower marks in the end of unit maths tests are immediately given a bit of help to get the topic nailed down, and there seems to be no opprobrium attached.
There are loads of state school boys of all economic circumstances in the 11+ intake, and a few in the 8+ cadre and the school in general is hugely more ethnically and economically diverse than the schools where we live (admittedly in Chelsea). I think that my DS's year at intake lived in about 15 postcodes and barely a third come from West London. This doesn't leave much scope for pavement sociability as everyone is nursing cars on yellow lines or dashing for the tube (there is a system to avoid tickets on the yellows) but the parents are generally very friendly. My DSs' pre-prep doesn't have one of the most glammed-up school gates but the presentation at the WUS gate is definitely more varied and has a longer scruffy tail to the distribution and no one cares a bit. On the one hand the school is relatively varied, on the other hand there is a strong common commitment and enthusiasm for education, inquiry and learning and this binds the community.
As you probably know the 11+ boys are in their own forms for the first year which probably gives a more gradual integration.
I can't speak to the Great School from experience, but as boys from the Under School make up close to half the intake, I would expect there to be a fairly strong cultural carry-through. I really don't think there is much scope for flunking GCSEs 
On organisation, it is my DS's main challenge too. I sometimes wonder if the more brutal sink or swim of Colet might have bludgeoned it out of him faster, but he is getting there and a small school is certainly an easier place to learn these skills. I hate to think how much he would have missed if he hadn't had the time to master his kit.
I suppose the Easter and summer French and Latin classes for 11+ are a bit of a downside but I imagine that other places require this too
Can't comment relative to your other choices , but I think WUS is a great choice for bright and curious little boys.