Hi,
as Jakob described, one solution is using iMac 27" logic board + utility to disable iMac's original display...
I would say this is more then else "soft(ware)" solution.
I went a bit further. I find out where the hack is ... missing communication with some sensors placed at the display's logic board. If Firmware (not OS) detect there is no respond at request then iMac reduce the performance on 10%.
To replace the missing sensors I used small microcontroller. Beside original display I removed also fan.
New box has dimensions 27x21x6cm, almost A4.
I extended my iMac with capacitive button-switch instead of power button, microcontroller is driving small 1sqr inch tft white/blue display showing the status of 4 on board LEDs and some other informations about imac. Both placed in front of mini imac.
The best comes ... I built second graphic card in the mini imac box (geforce 1080) and 512gb pcie ssd (Samsung 950 PRO with transfer rate 1.5gb/sec R/W) - using TB channel.
Mini iMac is fully loaded 32gb ram, 2tb sata ssd, 1tb pcie ssd, 512gb nvme ssd + nvidia geforce 1080, i7 at 3.5ghz and internal graphic nvidia geforce 780/4gb ... and new cooling system.
The difficult part was replacing missing sensors and fan.