Why is my Mac Mini not Turbo Boosting?
When you run the command sysctl hw.cpufrequency you are getting a constant that was given to the OS at startup.    If you want to get the CPU frequency "on-the-fly" you will need to poll the CPU.  That's what the Intel Power Gadget is doing.

As for why you are not seeing a performance increase with your specific application, it could be from any number of factors including whether or not the application is optimized to work on this particular architecture.   

Disclaimer:  Personally, I don't know much about ray tracing software, but from the little bit that I did read up on. I don't see why the calculations for the creation of the trace wouldn't be done on the GPU and not offloaded from the CPU.  This makes me wonder if this is not what's happening here

My suggestion here is to use something that is consistent across all platforms like Geekbench

Finally, Intel has this disclaimer about the Turbo Boost technology...



and...



So, as you stated it appears that your CPU is "boosting" but as to why it's not getting up to the full 3.5 could depend on any/all of these factors that Intel noted.