@KosmicRavioli Thanks for getting in touch (that's a great username).
Here is briefly how I was going to answer your first message, but then your second message raises some questions:
The Analog Caravan was my first PT6 simulation, and each successive aircraft has received significant improvements as I've learned what's possible in MSFS. While I would have to look at exactly what's simulated in the Caravan (I wrote the code almost four years ago), I am not surprised to hear some of these limitations. For instance, I know I wasn't simulating residual heat at the time (that debuted with the TBM), and I know my starting simulation was also not as refined, and produced the temperature spikes you're seeing (debuted with the Dukes and back-ported, if memory serves).
Now, your second post is more interesting, because all these features are included in the Turbine Duke. You should be well over the ITT limit with the power levers firewalled at 24,000ft. In fact, if we look at the performance tables in the manual, we can see by the shading that we become ITT limited somewhere in the neighborhood of 20,000ft. I have one possible thought, although it might sound a little silly. Have you checked the condition of the engines? As they begin to decline below 25%, their power output will decrease, which would probably also decrease ITT, and lead to a loss of airspeed.
If you're still having problems with the Turbine Duke, please feel free to send me videos or screenshots, and I can help you troubleshoot from there.