Well i thinks its hard to say as we aren't talking about 20% fuel economy but very small percentages. I can't honestly say yes without running it in perfect conditons before and after, which i haven't.
Regarding stability? Anything that improves air flow in an area which is left behind in terms of drag is always a good idea. The other point to make is that Audi wouldn't have fitted them to the most eco model for fun, it would have only been to make a difference inconjunction with the other mods they made to the 1.2 model. So in that area i think its worth fitting them, stability is probably better (if its possible to measure inaccurately from behind a steering wheel).
I know this sounds very vague, but i wouldn't make big claims, just that I understand the concept behind why Audi improved airflow under the car of the 1.2 and believe "over all" it can only help other A2's without the smoother underside fins and covers.