In various whiteblock turbo Volvos, we have consistently found that the improvement in fuel economy makes 95 octane fuel cheaper than 91. IE: The improvement in economy more than outweighs the extra cost at the bowser. Performance is noticeably better too.
In some of them, 98 octane petrol makes enough of a performance/economy improvement to be worth the extra price of 98. This is not always true, and depends on the exact vehicle.
Honestly, the only way you'll know is if you try it. Put a couple of tanks of each fuel through the car and see what you find - keep track of the economy on the second tank because it will take a while for the ECU to adapt to the different petrol and settle down enough to give you reliable figures.