I attended a specialized math and science program (MaCS) in the TDSB. It was gutted by removing selective admissions in favour of a lottery, precisely because of the report you've cited.

The "levelling" is real in Canada and good private schools often manage to skip multiple grade levels.

Funnily enough, I've seen the opposite in the USA. My highly driven American friends somehow manage to get entire associate's degrees before finishing high school, which is unthinkable in Canada.