The pilot stated that the engine was developing full power during the brake release and takeoff roll but that the aircraft would not accelerate to lift-off speed as fast as expected. Upon reaching 70 m.p.h. (61 kts.), and after a takeoff roll of 2,000 feet, the pilot further stated that he rotated and lifted the aircraft off the concrete runway but kept the aircraft in ground effect when it was unable to reach climb speed. The P.I.C. aborted the takeoff past the runway departure end by reducing power and lowering the nose and the aircraft impacted the ground in a grassy clearway inside the airport perimeter. Post-accident calculations reveal that the aircraft weight and balance was at or very close to the maximum takeoff gross weight (MTOGW), and within C.G. limits. Aircraft performance estimates, based on field elevation and nonstandard temperature and pressure at the time of the accident indicated a density altitude of 3,600 feet with headwinds of less than five knots.