I genuinely don't know Natfontana. DD2 is very bright and at greater depth but I would say more gifted in the maths area than in English so doing do well in HB stage 1 was quite a pleasant surprise. I do think prep goes a long way, my older DD is very bright too but was much less motivated and did much less prep and was a long way off passing stage 2.
But then there were a couple of her friends who have been tutored and didn't get through. I do think there comes a point of diminishing returns. Once they are at the master level on all topics on atom and are familiar with the question types it's more about reducing silly errors, and I did find she plateaued a bit over the summer and got less motivated as a result. We went away for about 3 weeks in total (spread out) over the holidays and she only did the odd 10 min test while we were away. She did quite a bit of self study in the final week of the holidays but that was her preference over holiday club. She stuck to school hours and then stopped at 3pm and had weekends off.
She is a bit of a geek as I said though. Maths club is her favourite club, and she'd often pick up puzzle books etc so I just switched these for GL books - i would leave them hanging around on the breakfast table and she would pick them up while eating breakfast.
Dd1 is a different beast- I would have needed to be much more directive with her and I'm not sure that would have sat right with me. I didn't know about atom learning then though and that might have made the difference. I'll never know.