I don't think the octane will change anything in the 2.5. It's designed for regular fuel and it won't improve on premium. And if it is any kind of difference it's too slight to notice. You'll end up wasting money in the long run.
As far as the throttle response, it won't change with the gas either. So save the money, and use regular. Just make sure you use trusted gas stations and you should be fine.