I can share a few insights from an outsider's perspective - I'm sure Chet's parents will be along soon to tell me where I'm wrong 
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We live locally, and our music service has a kid or two each year that goes to Chetham's, plus we have some exposure to the school through outreach stuff. The kids that it suits are very intrinsically motivated, and usually have a clear sense that they definitely want to be a professional musician. These kids love it and cannot think of anywhere else they'd rather be.
There are lots of kids who win national and international competitions, so you need a good sense of robustness to manage that if you're in the middle of the chasing pack. I do know one kid who was a chorister there, and not invited to stay beyond Y8, as they were not thought to be of the required standard on their instrument - for context this young person was accepted immediately into a conservatoire Junior Department and is now at a conservatoire doing a BMus, so it's not like they were still fumbling through Frere Jacques.
My DC go to junior RNCM. They definitely didn't have the drive for Chet's in the past, and maybe not the talent either. (My elder DC is reaching that level of commitment now at 16, but has decided not to look at it for sixth form even though it might work now). RNCM is surprisingly relaxed and nurturing, and about personal growth. A lot of the staff my DC have encountered from Chet's through outreach, NCO, NYO etc seem far more focused and business like than their RNCM counterpoints, which I'm sure works fabulously for really driven kids. And we've only met a handful of staff, so our experiences may not be characteristic.
In short my view from knowing a few kids there, and meeting a few staff is that it is the perfect place for some kids, and possibly a bad fit for some others, depending more on temperament than talent. If I had a kid who was keen to go, and they weren't already attending JD I'd look at that first if geographically possible, as might be a gentler first step.