Ok, here we go:
1. Assure you use BIOS V 3.0 or 3.0a
2. Set Bios Boot to legacy (not uefi), disable hyperthreading.
3. Install Yosemite with this guide:
http://www.tonymacx86.com/yosemite-desktop-guides/144426-how-install-os-x-yosemite-using-clover.html
3.1 Create USB Stick with the guide (Step 2) - Copy uncompressed EFI Folder from EFI-USB.zip to stick, overwriting the EFI Folder there.
3.2 Boot with USB Stick (F11), it takes very long until you see the GUI. At least a few minutes. Install Yosemite on your Main HD (HD must be GPT Formated, use Disk Utility in Installer (Partition Tab and down there is "Options". Select GPT instead of MBR)
Use Guide Step 3. There should be a second reboot with the usb stick. Don´t forget to press F11 and choose the stick a second time!
3.3 Post Install - Use Guide Step 4 UEFI Mode. Before reboot copy Folder EFI from EFI-MainHD.zip to the EFI Partition, overwrite the existing Folder. See hints down below howto mount EFI Partition.
Open Clover Configurator. Load EFI/EFI/CLOVER/config.plist.
Go to SMBIOS TAB, Press the Magic Wand Button on the right. Choose IMAC 12,2 and generate a smbios.
Change Memory Channels to Quad and 16 Slots.
3.4. Reboot, enter BIOS. Under Boot, set Legacy to UEFI. Choose "Add Boot Option". Choose your Disk, goto EFI/BOOT and add BOOTX64.EFI. Give the Option a name and press "create"
Set the Option as first. Try to boot, it should work at least if you have not more than 16 Cores. That was the limit I encountered.
Install Kexts of Kexts.zip with Kext Utility.
Reboot, and you should have Network.
Kext must be installed every time you update. But it is faster to go to Terminal, issue cd /S/L/E/IONetworkingFamily/Contents/Plugins/AppleIntelI210Ethernet.kext/Contents/ and do a sudo vi Info.plist and exchange the string 15338086 against 15218066 and save.
Then do a Kext Utility full service.
Hints:
You can mount EFI Partition so:
Enable Disk Utility Debug: In Terminal: sudo defaults write com.apple.DiskUtility DUDebugMenuEnabled 1
Restart Disk Utility: In the Debug Menu choose: "Show every partition"
You see then the EFI Partition of your HD. Mount it.
Get Kext Utility:
http://www.tonymacx86.com/downloads.php?do=file&id=210
Get Clover:
http://sourceforge.net/projects/cloverefiboot/
Get Clover Configurator:
http://mackie100.webwacemaster.altervista.org/
Put those Utilitys on your USB Stick.
Good luck!