My DS went to Sylvia Young full-time school for a year before deciding it wasn't for him and moving back to an "ordinary" school.
I was pleasantly surprised to find the academic teaching was generally very good, but the number of subjects on offer was much more restricted than at most secondary schools - only one MFL, for example. The vocational teaching was unexpectedly patchy; very good in dance, OK in singing and pretty ordinary on the acting front. Students who were particularly focused on one discipline tended to take extra classes outside the school - so the serious dancers would attend local dance schools or associate programmes as well, the serious singers would have private singing lessons outside school rather than just relying on the school's group lessons, etc.
All the students were automatically represented by the school's own management agency. Because of this, a lot of the logistics of professional auditions and work were easier - for example, auditions were often carried out "in house" during school hours, and the agency would liaise with the school over things like permission for professional work. Being at the school is definitely not a guarantee of professional work or even audition opportunities. The children who did lots of auditions and were cast in productions tended to be those that already had good professional credits before joining the school.
The school definitely suits a particular type of child - robust, self-confident, sociable. There is quite a high student turnover; some start and then families find they can't sustain the fees or long commutes, some decide it's not for them. The school tends to manage out students that require anything other than a light touch on discipline. The focus is very narrow - performing arts only and no variety of extracurricular activities, which some children love and others can find a little claustrophobic. DS eventually decided that he wanted more variety than the restricted curriculum and lack of extracurricular activities could offer. He's much happier in a school with a broader range of opportunities, continuing performing arts classes outside school (with the advantage that we have more control over teachers/providers).
However he has plenty of friends who are happy and thriving at Sylvia's, as well as Italia Conti, ArtsEd and Tring Park. I think it is hard to know if it will suit your child until you try it.