Using 98 octane fuel is not going to affect your engine or mileage, and if you could buy it, it would be a waste of money.
93 is slightly better than 91, but I wouldn't worry about it. I've been using 91 for the last year and it has not affected my car in anyway. I still have plenty of power, and no problems.
http://986forum.com/forums/showthread.php?t=3624&highlight=high+octane+race