"Mainstreaming" was a new concept in education about thirty years ago. So there's almost certainly plenty of academic literature on its outcomes...with the usual caveat that it would be the sort of literature likely to have research to support conclusions versus vice versa.

But really, this is case by case -- or rather individual by individual according to context. If the educational institution is trustworthy, then their recommendation probably is in good faith. If the institution isn't, that's a core problem.

The outcome is going to vary more with individual teacher competence than the student makeup. A good teacher will better meet your child's needs irrespective of class composition. Bad behaviors happen across abilities.

My advice: make the decision at a fine grain of actual teachers and classrooms. If it clearly isn't working out, switch to something else as is your right. It's a long haul from kindergarten to college and it usually turns out alright. Good luck.