In my experience, high achievers become interested in topics only covered in higher grades sooner or later anyways. And once they do, they pursue them only out of interest and intrinsic motivation, and that tends to boost their motivation. So I don't think this affects high achievers all that much either way.

Almost by definition, school (and everything else) is designed for the top of the bell curve.

Now, maybe school plan design can move the median ability up or down, and I don't have an opinion on if this change here does that, and if so, in what direction. I do think it doesn't affect the high end much either way.