I have a daughter who used to hate math and now loves it. She loves MEP and Elements of Math (online) and is doing well. I waffled about skipping MEP 5 last year because she was doing so well but decided to do it after all because I wanted her to feel confident.
She rarely gets a problem wrong and easily finishes the lessons. We're 10 lessons from the end of the year and I'm trying to decide whether to skip Y6.
More interesting problems
Not spinning wheels covering easy problems
A bit of acceleration (I'd rather her be a year beyond where she is now)
Possible frustration if too hard
Losing the gains we've made in love of math if frustrated
We'll stick with MEP because it works so well with how she thinks. (I've read all of Lewelma's posts on using MEP secondary.) She doesn't seem annoyed that the problems are too easy for her, rather happy that she can do so well.