It depends.
I grew up out west in a city where it was rare to do FI. In that case the kids that went there weren't the kids that were struggling in school so in a way it acted as an unofficial bright/gifted magnet school. Kids that struggled in FI left or didn't try it to begin with.
I now live in a very bilingual city where FI is VERY popular so it doesn't quite the same effect. Some kids do opt out but we still have ~2/3rds of kids in FI.
My kids are grade 3 and grade 1. My oldest is gifted/LD and HATES french. He is very math/science oriented and I don't think languages come easy for him (maybe related to the LD stuff??). My youngest is untested and absolutely adores French. She is picking it up very easily and at least there is something for her to learn at school. For DS I would pick congregated gifted over FI in a heart beat but for DD (if she is gifted) it might get us by for a couple years but she does still complain about the repetition and that things are too easy. So much depends on the kid, the teacher and classmates. Our experience with differentiation in a classroom is that it hasn't happened despite what they keep promising.
