I do wish some posters would read what you said before posting their replies!
However, I have to say that I have found it to be fairly unusual for a 365N to fail its S/E height climbs. If it does it's usually a sign that there is a problem with the engine or one of the parameters. To eliminate one of the most common errors, it's a good idea to use a calibrated digital OAT guage for the height climbs, and then to circle at the mean height for about one minute to get an accurate temperature reading for that height. I also find it helps to carry out the flight early in the morning or in the evening to negate errors due to turbulence (a flight over water also helps in this respect, where possible). I try to start at a height which keeps me out of turbulent air and climb for 5 minutes to get good 'averaging'. When we have had engines which have (rarely) failed we have always then looked at the factors which may have caused an erroneous result - inaccurate OAT readings, inaccurate Ng and Nr gauges, incorrect basic weight and/or fuel calibration, 'air' leaks (P2 air, or heating/vent ducts which are leaking). I carry out ground and in-flight 'power assurance' checks iaw the FM as a cross check and look at the average rates of climb throughout the flight to see if over any 30 second period they have deviated from the 'graph' due to turbulence or my inaccurate handling. If everything has been double-checked and the aircraft fails there is only one thing to do with a 365N - ground it and wait for the new engine to arrive. There is no consideration given in the FM (as with some models of the S76, for example) for a reduction in MAUM for Cat A operations for below spec engines.
If I were in your position I would put the aircraft unserviceable in the Technical Log and if your Tech Manager refuses to accept that have a word with higher management in your company as to whether they value the services of yourself or the Tech manager more! His attitude is totally unacceptable if you are operating for an Air Transport Operation in the 21st century, and would never be condoned in my company.