Most of the Unichip tunes are programmed for higher octane fuel. The manual that came with mine says it was 87 octane on one map, and 93 on the other.
The only time you will gain power with higher octane is when the car is programmed to use it- more timing advance, etc. If the car is tuned to use it, and you're still using 87, there's a chance the knock sensor is retarding timing back to the point where you may actually have reduced power with your unichip.
My advice is to figure out exactly what the chip is tuned for.