I don't know what research you are pointing to. But I am pretty sure that if, say, a student can't pass a multiplication test, but you give that student an A+

I don't know what research you are pointing to. But I am pretty sure that if, say, a student can't pass a multiplication test, but you give that student an A+ anyways, and then let that student into a "top calculus class", that's going to hurt the student more than it helps. Ya sure the student gets to mingle with better students. But what you would usually get is, the student would end up being someone who mentions a lot of buzz words like "fractal integrals" just to fit in, but have zero idea what is going on, or what is being taught in class.