Mike
You have indentified the heart of your problems, no need for more guess work as your combustion chamber is in need of a replacement or a major repair job. I would have expected to see this type of condition only after say repeated multiple wet starts and with temps *****g out over 800c on running. I'd wonder if they are inconel if you are usually under 700c. A PM to Andy at Anglesey Turbines might give a few suggestions as he will give you an unbiased opinion, as you are not in the UK. He does offer a different CC which I have used on three of mine.

The figures on the exhausts was a very useful comparison since it was on 'like for like' basis (ignoring the CC condition). The usual temps I have recorded are 620c on tick over 45k falling to 565c on max rpm, so I'd expect you to be able to drop 70-80 degrees at max rpm. With other adaptations on one JJ1400 I was able to get 560c at 45k falling to 480c at 160k, much much cooler but thrust was down requiring further work and more restriction to the exhaust. Cooler temps also allowed higher rpms above 160k. Look forward to hearing how you resolve these immediate problems.

Paul