It depends.
I can think immediately of one central London private school which probably has a high proportion of SEN. It is less selective so takes kids who were not thriving in either local state schools or indeed very selective private schools. It also took kids with medical issues or those transferring from other educational systems. We know at least two very bright kids, there for specific reasons, who have done very well despite very mixed ability (albeit small) classrooms. I also met a mother who was so so relieved that her child was finally enjoying secondary after hell in a state school with little SEN support.
If your children are happy, I would look instead at your children's ability to maximise their potential. What are exit destinations like? Are some doing very well. Is school work differentiated? Happy children learn, but bright children also need to be stretched.
In terms of bright children having SEN, DD certainly did, indeed an astonishingly spiky profile with lots of top 2% and bottom 2%s. She went to a couple of very selective schools, with no obvious interest in gaming the exam process. Kids getting SEN support, if that is the right expression, was often a broad church. Dyslexics like DD, but also kids with ASD, indeed autism, chronic disorganisation, deafness etc. It would have been nowhere like 40% but children with SEN end up in all types of schools. One noticeable problem with bright kids with SEN is that they often develop good coping skills so it goes unrecognised, till say, they attempt four humanities A levels and start floundering. I understand the same can happen at University.
Rushes off before Bubbles starts hurling insults, as she did last night on another post.