The answer is that I don't know with any certainty the stock turbo tends to give out before people hit a power limit.
I don't think those have variable valve timing on the exhaust if it's the 120hp engine you have, so choose your turbo wisely.
You will also struggle with traction at those figures - the A1 is so light. I personally think the sweet spot for power on those is around 180hp.
There are guesses that the stock fuel system on the 150hp units are good for 280hp+, but I've never seen anything solid to back this up. I'd be very curious to know if you find a definitive answer to this question.