The manual is clear. Premium improves performance. Manual notwithstanding, it stands to reason that higher octane is beneficial for a car with 12 to 1 compression. There are two questions the manual and common automotive sense don't answer. First, does premium solve any of the complaints like shuddering? Some on this forum have reported that it does. I use premium and have had no shuddering or other issues. Second, is the added cost worth it? I doubt premium's added cost is worth the performance gains or MPG gain (if any -- regular may give better MPG).
So as I see it, premium is worth a try if you've had shuddering or other performance issues, or if you're a leadfoot who wants to see whether there is a power improvement.
Once I have about 4k miles, I intend to switch off premium to see how regular compares in the fuel economy department as that is my principal concern.