ATN,
1) 1 degree of OAT drives up the operating temp of the engine about 3.5 degrees, based on the thermodynamics of the way an engine works. It is mostly linear, but slightly different for some engines. Look at the power assurance chart (which might show target T5 vs Torque), and simply enter it at the same altitude, same power, but at two different OAT's. The ratio of OAT to T5 is usually about 3.5 or so.
2) The answer Ghengis gives is that the performance of an engine can go below spec, but the engine might not dangerous to operate. That is what "airworthy" means. Most engines can degrade below spec cheerfully, to perhaps -10%, before they get non-airworthy. If the cause of the power loss is erosion or dirt, and not busted parts inside, the engine will probably degrade further until it can't accelerate properly, and it starts to stall when you ask for power increases. That is because those raggedy compressor blades no longer smoothly transport the air to the combuster, so the engine flow becomes unstable. Most degraded engines worry the engine manufacturer because of this possibility of stalling on acceleration, and that stall sets the minimum safety point. I must say, though, that most engines have no such published permission to operate below the min spec point. When performance is a problem, the aircraft manufacturer and the engine manufacturer collaborate and get the airworthiness authority (CAA, FAA, or the military service who bought the machine) to agree. They agree by publishing such a procedure in the flight manual and the maintenance procedures. Only then is this theory we describe allowable.
3) when you say the gross weight was reduced in the aircraft in question, from 4000kg to 3600kg, was this done using a factory (flight manual) procedure? If so, it is the kind of "below speck" procedure I discussed above. If it is an ad hoc procedure made up by somebody else, be wary. only the regulatory agency who approves the flight manual has the ability to wave such limits.