Vinh -
I don't think cutting the output nozzle off will produce more power. The pump is only going to pump as much water as it can. The nozzle just focuses the output water and creates more of a streaming effect. Think of a water hose that is flowing at full pressure. Without anything on the end of the hose, the water just gently flows out of the end. If you put your thumb on the end of it, creating a nozzle effect, the water coming out is focused and has more direct pressure.

The best thing to keep up the performance of your powerheads is to take them out every so often and clean them. Junk gets sucked into them and algae grows on them. This can slow the impeller down and clog up intakes/outputs.