> By a "superintelligence" we mean an intellect that is much smarter than the best human brains in practically every field, including scientific creativity, general wisdom and social skills.

To me, that's a really good definition. "Much smarter than the best human brains in practically every field". It's going to be hard to weasel around that.

> Once artificial intelligence reaches human level, there will be a positive feedback loop that will give the development a further boost. AIs would help constructing better AIs, which in turn would help building better AIs, and so forth.

This depends a great deal on what the shape of the processor-power-vs-better-algorithm curve is. If the AI can get you 1% better algorithms, and that gets you an AI that can in turn get you 0.5% better algorithms, and so on, then yes, you're still getting "a further boost", but it won't matter much.