It's obviously not like: 10% lower octane is 10% less timing advance, 10% less power, and 10% more fuel consumption. I don't think it's even possible to calculate beforehand how much you will benefit or not from lower or higher octane fuel, even if you precisely know how the ECU will react. And that's the thing: It heavily depends on how the ECU deals with the situation.
Ideally, your ECU would change its spark advance just enough to cope with the lower octane fuel, while still giving you the best performance possible. Just like with higher octane fuel. But in my experience, it's only the top notch ECUs (read: real expensive cars) that do that. In that case you'd have to test it on the dyno to find your performance and fuel economy gain, I don't think there is another way to determine it. Keep in mind that every ECU can react differently, and results of other people may not apply for you.
It's possible your ECU just firmly pulls down the timing advance, like 10 degrees, when it detects ping. All the profit is lost then I think.
Not even all cars have knock sensors. If an engine is designed for a specific octane rating, I don't think you can improve things with higher or lower octane fuel. It's certainly not worth the great time investment of adapting the engine for that. As far as I know higher octane fuel is only used in sports cars that have their engines designed and optimized for it. (Higher being 98RON, lower 95RON, the only ones widely available in the Netherlands.) Next to spark advance, there's a lot of additional design parameters that you'd have to change to really make use of more spark advance.
In short: Your average Opel Astra engine is in all probability not susceptible for adaption to really benefit from different octane rated fuel. Lower grade just results in loss of power and increased fuel consumption if you don't adapt. Higher grade will probably have no effect.
I don't have any sources or hard evidence to back this up, but I'm speaking from personal experience.