People who study a lot study a lot for the SAT, people who study little study little for the SAT, and in between people do an in between amount of SAT. That somehow seems like a good starting point, which we'll probably be stuck on given evidence will be hard to come by.

FWIW there were kids in my kid's class who had three tutors and studied every day over the summer holiday (the test is right after). They would have stopped if it was so obvious the benefits stop past some level. Just as there will be people who think it's a great idea to put 1000 hours into it, there's going to be people who think they'll just show up on the day and see how it goes.

Edit: seems this wasn't a direct descendent of the the thread I thought it was, about the UK 11+, but the point is the same.