Mutt, in answer to your question, yes, there is a performance difference when using an assumed temperature of 50 deg versus full rated thrust when the actual oat is 50 deg.
For example, on a 50deg day for the CF6-80, the N1 is 106.3%. On a 25 degree day, derating to 50 degrees gives a figure of 102.1%. Why? Because even though you have derated the thrust, the fan still has to spin in the air density of the 25 degree day. The EGT to spin the fan at 106.3% on a 25 degree day would exceed the Takeoff thrust EGT limit.
Sorry to get off topic, but the question was raised...
