In my case, Ds was tested for a learning disability and that's how it came up - they profile everything as the learning disability shows as a spike.
We are having testing for dd2 (yr5). In her case for dyslexia. Her reading has improved, but still slower than siblings, but spelling and writing still around yr1 level. She is good at maths, verbally very articulate, great at musical composition, a great story writer.
She may well fail her 11+, not overall - because she is likely to pass her maths and non-verbal reasoning, and possibly verbal - as she can spot hidden words because she isn't always seeing the whole word anyway. If she fails just one paper (in her case English) she will not pass, even if she got top marks on the others. Even at appeal she is likely to be deemed 'not of grammar school standard', because of concern that she won't be able to keep up with the pace due to dyslexia.
Fortunately our 'secondary modern' options are outstanding too, but she will lose her peer group of friends as the top 25% disappear off to grammar school. She will also at age 10 have failed in something and will see her friends and family access schools and opportunities denied to her. Passing might be more disastrous as there is less SEN support in the Grammar school (according to school who said that they don't really cater for children with dyslexia).
Grammar schools don't select the actual top 25% on IQ, they select all rounders whose parents have sent them to private school and tutored them since yr2 who will need less input across all subjects. With a spiky profile var12 your ds might not have passed the 11+ and so go to a school with the top set stripped away.