ACCORDING TO WITNESSES, THE ACFT WAS ABOUT TO TOUCHDOWN FROM A NORMAL APCH TO RWY 21, WHEN IT ENCOUNTERED VARIALBLE WIND CONDS & BALLOONED TO ABOUT 70' AGL. THE PLT INITIATED A GO-AROUND. SHORTLY THEREAFTER, WITNESSES OBSERVED A PUFF OF SMOKE EMANATE FROM THE ACFT, THEN THE PLT TRANSMITTED THAT THE FLT WAS 'GOING DOWN.' SUBSEQUENTLY, THE ACFT COLLIDED WITH TREES ON WOODED TERRAIN ABOUT 1/2 MI FROM THE DEP END OF THE RWY. AN EXAM REVEALED A LACK OF POWER SIGNATURES ON THE PROP; EVIDENCE OF SOOTING WAS FOUND ON THE SPARK PLUGS (INDICATING A RICH MIXTURE). TESTING OF THE TURBO-CONTROLLER REVEALED EVIDENCE THAT IT HAD BEEN ADJUSTED TO A LOW SETTING; THE TURBO WOULD ONLY PRODUCE A MAXIMUM AIR FLOW EQUAL TO HIGH CRUISE POWER. THE ENG MANUFACTURER RPRTD THAT THIS SITUATION COULD PRODUCE AN OVER-RICH MIXTURE, PARTICULARILY IF THE THROTTLE AND/OR MIXTURE CONTROLS WERE IMPROPPERLY MANIPULATED. THE PILOT LACKED EXPERIENCE IN ACFT WITH TURBO-CHARGED ENGS & HE WAS NOT TRAINED IAW THE COMPANY TRAINING PROGRAM.