Peter...not what I meant.
The central London education landscape seems to have changed a a lot, even in the few years since we went through it. Or perhaps DC were lucky to have gone to a relatively unpushy prep.
Lots of prepping for pre-tests, then lots of prepping for the Challenge, often both inside and outside school. In contrast we more of less drifted through the process. DCs Prep were also clear that they only supported individual candidates for the various scholarship exams if they had a realistic chance of winning one, which is in quite strong contrast to other schools where some parents are keen to see their children in a "scholarship class".
I assume schools see this, hence recent comments on another thread about Eton looking beyond the traditional Preps for good candidates. Similarly I understand Colet Court has revised its approach to help relieve some of pressure on their pupils. (A lasting memory is seeing a Colet 9 year old with his ever present mother wheeling an air hostess suitcase containing his homework, as he dashed from his grade 8 music to his national level sports training.)
What I am trying to say is that the "Challenge" should form an interesting challenge for the right candidate. Interesting though, rather than super-hard, and the extra work should be both maneagable and fun. Some very very bright boys enter through the Challenge, but others, as Shegot suggests, appear to have been over pushed.
Education is a long haul. Our observation is that students at good Universities are expected to work extremely hard. Some children might have had 3+, 7+, 11+ pre-test, 13+ CE, 16+ GCSE, 17+ AS and 18+ A2. DS did Common Entrance which was pretty dull and not over challenging, or rather for him French and English were challenging enough. I don't think it made any difference thereafter other than he probably would have taken his French GCSE a year early. In sixth form he was in the same maths set as a couple of scholars, whilst others had maintained their early promise and were in higher sets. The big thing, and the real joy of Westminster, is that he has enjoyed his education, and has gone to University well prepared but not over stretched. Inevitably he has seen a couple of peers, perhaps facing higher expectation, buckle under the pressure on the way. And at University he can see fellow students, especially from the parts of Asia where heavy tutoring is the norm, come unstuck as they either struggle with the academic challenge, or simply struggle to find self-motivation once away from parents.
In short OP. If he has a realistic chance of gaining a scholarship, go for it. He will probably enjoy the preparation. If not, don't bother. (Unless you think there is a good chance of a Coronation wihtin the next seven years and your DS likes ceremonial stuff.)