Tiggy - I reached exactly the same conclusion for my DD as that mum you knew.
She has always been right at the top academically, went to top academic prep and was in top 3 in class for just about everything. However, her passions have always been writing, music, dance and drama. Her dream is to write and star in a West End musical! She did great academically at the prep and had lots of friends but I always sensed a bit of anxiety and she didn't seem entirely happy.
We have one of the most sought after super selective state grammars in our town. Hysterical competition, many of the girls at the prep sat entrance test, only a few each year gained places. So it was assumed by many that I would be sitting DD for this. Looked around on open day, talked to teachers. Very clear there was a massive emphasis on academics, especially science and maths, no interest in creative arts, parents who get their DCs in the school don't want it. They see creative pursuits as frivolous distractions from the serious business of gaining aa string of A*s. I just knew my creative DD would shrivel in this environment.
In the same town (luckily for us, only a short walk away) is very highly performing and popular girls comp. Lovely atmosphere, great for creative arts. I just got a very strong feeling DD would thrive. It does have streaming from the word go, though. But it has a reputation for being the second choice for those who do not make it into the grammar (which is about 99.9% of the local population! No catchment area).
I made the decision to send DD to the comp and not to sit her for the grammar. The parents at the prep cannot comprehend that anyone would pass up the chance of a place at the grammar, so just assume that she sat the 11+ and i just don't want to admit she failed!
Do I care? Hell, no! My DD is absolutely thriving and is happy and relaxed in a way she has not been before. Her academics are flying (7a for English and maths, 6s in most other subjects) and she has loads of time for all her artistic pursuits both in and out of school. Best decision we ever made!
Interestingly, DD tells me that several of her friends who sat the test actually put the grammar as second choice after the comp as they were using it as a back up in case they were out of catchment for the comp.
Anyway, sorry for the long ramble, but I agree wholeheartedly that just because a Dc is bright enough to gain a place it does NOT mean that the selective education available at a grammar will be right for them.