If you have a Z97x and a i7-4790K then you'll want to use iMac 15,1 for better CPU power management.
However you'll get a black screen when you first try to boot with it.
So follow these brief instructions to get you started and don't reboot until I say so.
Also this will break you iMessage and Facetime so you'll need to fix those again after all is said and done.
Follow this quick guide: (Thanks to vinnethp for these instructions, full guide link listed below.)
Log out of iCloud by going to System Preferences/iCloud and "Sign Out".
Using Clover Configurator, mount your EFI partition, and open your config.plist in Clover Configurator.
The config.plist file is in EFI\EFI\CLOVER folder.
Go to RT Variables and change ROM to UseMacAddr0, BooterConfig to 0x28 and CsrActiveConfig to 0x67.
Go to SMBIOS and select the magic wand button then choose the iMac icon and change to iMac 15,1 and press the "Shake" buttons a few times each and click OK.
Copy your new serial number go to the webpage
https://checkcoverage.apple.com.
Paste the serial # into the "Enter your hardware serial number" box. If you get the "We're sorry, but this serial number is not valid. Please check your information and try again.", you're good to move on to the next step.
(I know this sounds stupid but it means that nobody else has registered a device with that serial number yet. Which is good.)
Open terminal and type "uuidgen" to generate the system SMUUID and press Enter/Return button. Do this several times to generate random UUID's. Copy the generated UUID and paste it into the SMUUID field of Clover Configurator.
In Clover Configurator, Click on File then Save and quit Clover Configurator.
(To not get a black screen when booting with iMac 15,1)
Next go to this
page and download the script file, "fixit.sh", open Terminal run the script file following the instructions.
(Thanks to MacOrDie for the script file)
Quit Terminal.
REBOOT
After you've rebooted check "About this Mac" and you should see "Mac (Retina 5K, 27-inch, Late 2014)".
Hooray!!! You're now on iMac 15,1 system definition.
Next go to this
guide and finish configuring your SMBIOS using vinnethp's instructions.
Pick up after the "Copy the generated UUID into the SMUUID field of the Clover Configurator."
Then go to
this guide and follow the instructions to create a new SSDT.aml for your new current SMBIOS settings.
Finally go to
https://www.icloud.com/# and login with your Apple ID and go to "Settings" and remove any devices that you don't use. Remove any computer's you don't use by checking the serial #'s and keep the device that has the new serial #.
(If you own iPhones and other genuine or hackintosh Mac's that use the same Apple ID, don't remove those).
Do the samething by going to
https://appleid.apple.com/#!&page=signin.
HERE'S THE RUB!!!!
If you use iMessage and Facetime, the above changes to your system will break those apps.
Follow the fix guides on this site to get those straightened out. I don't own an iPhone, so I don't use those apps.
Now whenever you take a new OSX update, you'll break the script and when your system reboots, you'll get the black screen.
So my suggestion for that is this, and I know it's a pain in the a$$, but if you want to use iMac 15,1 then this is what you got to do.
First go into Clover Configurator, load your config.plist and take a screenshot of RT Variables and SMBIOS and save them in a safe place like iCloud or Google Drive, MS OneCloud because you'll need to re-enter all this information if you want to upgrade.
Log out of iCloud first.
Next change your SMBIOS back to iMac 14,2 using Clover Configurator, save and reboot.
Then do the OSX update, and once everything is all up and running, open Clover Configurator, load your config.plist and change back to iMac 15,1 and re-enter all the serial information from the screenshots, save and quit Clover Configurator, but don't reboot yet.
Next re-run the fixit.sh script.
REBOOT
After you reboot, generate a new SSDT using the above guide.
Log back in to iCloud and go to the websites also to make sure there are no unwanted Mac's in your account.
Check to see if iMessage and Facetime still work, if you care about having those.
Good Luck