The pilot overran the departure end of the grass runway during a precautionary landing following a partial loss of engine power. The 1,393-hour commercial pilot reported hearing a loud noise while in cruise flight, followed by a partial loss of engine power. The pilot attempted to land the high performance single-engine airplane on a 1,400-foot-long, by 75-foot-wide grass runway. The pilot, who had accumulated a total of 320-hours in the same make and model, reported that he "elected to set the plane down, knowing I would go-off the end of the runway." The homebuilt airplane overran the departure end of the grass runway and collided with trees and terrain. Both wings sustained structural damage. The pilot exited through the canopy with minor injuries. The pilot reported that the 260-horsepower engine had accumulated a total of 265-hours, with 7-hours since its last inspection. The reason for the partial loss of power could not be determined.