CS, it should be possible to get full performance figures from the Bell helicopter company in support of your investigation.From your figures the aircraft would have been `operating`,engine/rotor performance wise at a DensityAltitude of approx. 10500 ft, but Pressure Alt of 7000 ft..It would be useful to check records of POWER Checks carried out on each engine,and the condition and rigging settings of the rotor blades,as these will also have an effect on `actual` aircraft performance..Like wise ,engine instrumentation calibration ,torque/tit/etc.,and rotor governor settings.
I am not familiar with the -412,but have spent a few years as a maintenance test pilot.Do you do engine compressor washes regularly , and power performance checks...?
Hope you get your answers..