higher-octane fuel will lean out the mixtures, except as the the octane level is the fuel's rating of resistance to detonation it will probably be safer for the motor.
However, using a higher octane fuel without a tune may not see any gains because the burn rate will be reduced and the motor will not be running optimally. Best bet, just run 98octane what your car was tuned with.
If you really must, an amateur way of adjusting ignition timing to get the most out of a higher octane fuel is done by moving the CAS anti-clockwise slightly, test driving and repeat until you reach an acceptable knock level i.e. 20-30. Or read up on paulr33 PFC guide.