thats wrong, don't install your addon kexts to /System/Library/Extensions
1.3 GHz minimum frequency is per Apple's design. That is the same on Skylake/Kaby Lake MacBook Pros, might be like that on older/other platforms too.I've mounted the EFI-Partition, installed Clover, and then replaced the "Clover" Folder on the EFI partition with the Clover Folder from the GitHub Repo from the dark void, I've also installed all Kexts from the Folder with the Kext-Utility to the system drive. Maybe not the best solution but it works for me and at the moment I don't have the time to completely understand these clover magic, the last hackintosh that I build before this Dell-Project was 10.6 or 10.7 and the bootloaders at this time were not so powerful
Also I've found a safer way to under-volt my CPU/GPU on the MacOS Level and not on the bios level:
This guide works pretty good on my XPS 13 8th Gen, also the risk of bricking the system is greatly reduced
(This method could also used to find the stable parameters for the bios method)
I think the power Management on the 8th Gen chips does not work perfectly now. The idle frequency in Mac OS is always 1.3 GHz and the CPU consumes around 1.5-2 Watt of energy, under windows and linux the idle frequency is 0.8 GHz and the CPU with a power consumption around 0.8 Watt. With better powermanagement it should be possible to save one more watt in idle...
I've followed the general laptop guide to enable Bluetooth, it mostly works, but luckily I don't need it