Stay with the Premium, your engine requires a higher octane rating. If you really wanted to, run the vehicle down to just about empty and throw in a couple of gallons of the other stuff just to see the preformance drop. I wouldn't do this though because you are possibly setting yourself up for pings, knocks and preformance drops, not to mention possible engine effects later on. The money you will save on using lesser grade gas, will be used on engine repairs if you do this.
As a side note, check to see what the manufacture requires on octane rating and check the other grades (i.e regular, high or whatever the gas stations are selling) and see if one of the others falls within that rating and use it.
off topic: how do you like the RX-8? I had a RX7 (85) and loved it, so did the tree that took it out when I was dodging a car coming out of a driveway.