Well... here is my feedback :
I'm chinese, but born in France.
At home, we lived with my grand-parents who spoke only chui-chow.
In the beginning, at school, it wasn't so easy because we were not able to communicate so easely with other children, especially for my big sister as I had her to speak french to.
But children are really adaptable, so we manage. But I have to say that even so, I know that my level in french isn't so perfect. I can see the difference with my husband who is french-french, fully french !

Of course, I'm fluent, but well, at school I always had difficulty in french class... And now sometime, I have to think before finding a word. Is it because I'm not in a french environment anymore ?...
But something for sure, I learn other languages much easier than other kids. Still do. Since coming to Hong Kong, I have learn quite enough cantonese to have small conversations and I never did have cantonese class.
2nd example : My niece.
The situation is even more complexe : she was born in Hong Kong from a french-born-chinese and a Japanese. My parents spoke to her in mandarin. And she goes to the french international school.
Now, she speaks French, Japanese, Mandarin, Cantonese and English !
But even in french, which is the language she use the most, she hasn't got the perfect level. She is fluent but she hasn't got the level that my husband's niece who leave in France, have.
My nephew, for some reason, doesn't want to speak in Mandarin and Cantonese. But his level in French is better !
So in conclusion, I would say that it does seem that the more languages you have, the less you will be able to reach perfect level.
But do we need to have a perfect level when we speak so many languages ? What is the use of languages after all ? If people understand you, it should be enough isn't it ?
Well, except if you want to be a diplomate. But then, maybe you would work harder on languages to reach a better level ? It's a matter of choice.
For my part, I'm quite happy to be able to speak many languages, and I do believe that we are going in a world where being able to communicate in different languages is a must.