Depends largely on the car.
I have done a bit of work to my car and have had it tuned to advance the cam timing to take advantage of the extra purity in the petrol.
In a normal car, with U91 you may experience a small bit of 'detonation' due to the low octane rating, especially when the throttle is "opened quickly
" but the car generally compensates for this by retarding the cam timing a little, this reduces power output.
For a regular car not tuned for 98octane, there will not be a significant difference. The biggest difference is between U91 and U95, before my car was tuned i put in 1 tank of U91 and my car went like shit, then put in U95 and it went much better. Then put in U98, not much of a difference between that and U95. I have used nothing but U98 for the last 2 years
I'd say fill your car with 95 for the next 5 tanks (will be at most a $4-$5 difference for a tank) and see if you notice a difference. Should get a little bit more power and a little bit more travel out of it