Unless it's tuned specifically for high octane (higher than 93) you shouldn't expect to see much, if any improvement beyond 93/94. It's not surprising (if you're at sea level) using 93+ instead of typical 91 made a noticeable improvement. Are you in California? I've only ever heard Cali 91 is about as good as pisswater...
Higher octane doesn't directly mean more power, just more thermal stability (resistance to knock). This means higher octane allows the engine to be tuned more aggressively before heat/knock become an issue again, which in turn means more power. The ECU is tuned to use pump gas, 93 being ideal because it's what is typically available. Higher octane means it's just easier for the ECU to stay in the happy range of it's knock compensation maps.
The general idea is to use the lowest octane rating that your engine is happy with. Unfortunately, our little lumps only like the expensive stuff.