As turbine exit temps increase to the limit, the increase in engine life used increases exponentially. If you can get hold of the Harrier Engine Life graph, it shows the problem well.
An increase of tgt by only 10 deg C may increase damage by a 100, or even up to a 1000 times. Therefore, the lower the temp the better. So, for engine life, it's often better to use a slightly lower temp for significantly longer. (This is not the case for piston engines, however.)
The R-R CTP should be able to give chapter and verse on this, send me a private e-mail and I'll forward it on.
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