I’d also like to achieve the same thing. We had a customer drop his 5k Display 27” and decide to replace the entire computer rather than the screen. I now have an upgraded 2015 27” I’d like to use as a desktop (put into a case). I’m experiencing the same dirt-slow throttling. Macs Fan Control or TG Pro will manage the fan speeds, but the only solution I’m aware of is to use the board/sensors from a screen to trick the machine into thinking there’s a screen attached. Let me know if anyone finds a fix to this, as this machine would be a great desktop computer for my purposes if I could just get around the CPU throttling. Thanks!