What you need to do is do some research on how your engine is programmed from the factory. For instance, my Dodge Hemi is programmed for 89 from the factory, but after digging around, and after asking some service guys I know who work at Dodge dealerships, I found a tech service bulletin put out by DC that states that the fuel injection programming that the hemis have from the factory will advance timing a bit farther if 90 or 91 octane gas is used, thereby making more power. However, the computer won't advance timing beyond a certain point, so adding gas that's higher than 91 octane won't add any additional power.
Nissans, I know, are the same way. The 3.5L V6 is tuned to run on premium, but you can add either regular or midgrade-as long as you're willing to deal with a 20-25 hp loss. The computer just adjusts the timing based on knock sensor data to account for the lower octane.
The important thing to remember is that no manufacturer's programming will keep increasing timing in response to adding octane-at some point, the computer stops adding timing, regardless of how high the octane rating of your fuel is. So, although premium around here is 93, my computer has its timing advanced as far as it'll go once the blend in my gas tank was 91 octane.
The best places to look and see if your engine will benefit from using a higher grade gasoline are internet bulletin boards that deal with your engine. Look for postings where people have dyno'd their engines with different grades of gas, to see if adding octane added power. You'll find that info fairly easily.
Fitz