[Xibalba]

Language only inhibits/influences thoughts in as much as you forget the difference between word and that which you speak of. I imagine, though this is certainly a rabbit out of my hat, that growing up billingual helps you because your ability to express yourself internally is "doubled" (not exactly accurate word, but whatever).
Also, the flexibility of being able to swith from one language to another allows you to formulate your own internal language. I do not mean a completely new alien thing inside, but that you can sort of formulate codewords, expressions that simply free what it is you are thinking, and so, nearly literally, let you think "outside the box".

However, I'd be wary of using "intelligence" as a key-word here. Though I agree intelligence is enhanced (and may be a pre-requisite for the succesful internalization of other languages), I think the word carries connotations that make this discussion about something else entirely.
I'd use something like "brilliance", but not in any judgemental way, more like how artists think internally, or great scientists. They have a way of expressing themselves internally that more often than not make them sound completely wacko, but anyone can realize that the problem is that they can't translate what they think inside to the people outside. I think that's what growing up billingual helps you with, the ability to start thinking less restrained, with the added bonus of giving you more "linguistic" tools as to how to explain yourself with other people.