I'm not aware of that research.
Our results cover many years here in New Brunswick. We had a grade 4 entry point in New Brunswick at one point. The research that I've done with the Ottawa-Carleton District School Board and other school boards shows that the early-starting students perform the best, followed by those in the middle around grade 4, followed by grade 6, which is what one would normally expect on overall proficiency results.
Of course, it depends who those students are. If you have some kind of selection going on in grade 4 and grade 6, your results may be comparable, but you have to remember that you're not talking about the same kinds of populations. You're talking about much more select groups in those cases.