Prior to landing, the pilot overflew the airport and checked the runway environment. A friend of the pilot who was in another airplane, landed first on the grass runway, which was 2,160 feet long by 60 feet wide. The pilot then attempted to land on the same runway. The pilot aborted the first approach after he decided he was too high and fast. During the second landing attempt, the pilot touched the airplane down on the main landing gear about the mid-point of the runway, held the nose wheel up, and applied gentle braking. He stated the airplane slowed, but not at the rate he expected. He considered aborting the landing; however, he felt he would not have enough room to clear the trees at the end of the runway. The pilot then hoped a short overrun area would help stop the airplane, but the long grass concealed a 20 to 30 foot steep slope. The airplane exited the end of the runway, went down the slope, crossed a gravel road and impacted small trees, sustaining substantial damage to the fuselage and wing spars. After the accident, the pilot realized the runway had a significant down slope which he could not identify during his fly over prior to landing. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.