I have 2011 iMac.
I did the following.
1. Installed Windows 10 from bootable DVD as per Apple's procedure. I had Windows 8 DVD to fool OSX bootcamp app into going ahead with the process. Then before boot, I quickly changed to Windows 10 X64 DVD.
Installed Windows 10 as normal.
2. Download Bootcamp V6 from links posted in this thread. Installed a minimal bootcamp (bootcamp.msi only without any device drivers) I edited bootcamp.msi V6 using ORCA to remove installing restrictions.
3. You will need to extract the following drivers from Apple directory of Bootcamp 6 source and point the driver update wizard to those corresponding folders.
a) Facetime Camera = AppleNullDriver64
b) Bluetooth Device = AppleBluetoothBroadcom64
c) USB Input Device = AppleKeyboardInstaller64
d) USB Input Device = AppleKeyboardInstaller64
e) USB Input Device = AppleKeyboardInstaller64
(Point all the USB Input devices to keyboard drivers if you are using Apple keyboard and magic mouse.)
f) Bluetooth mouse = AppleWirelessMouse64
g) High Definition Audio Device = CirrusAudioCS4206x64
h) High Definition Audio Device = AMD (From AMD 15.7.1 WHQL Driver Package)
i) Grab AMD 15.7.1 WHQL mobility driver from AMD website. Extract them and point Windows driver installer to that folder to scan and auto install AMD 6700 GPU driver.
j) Intel Management Engine driver gets auto installed from Windows Update.
This is all that is needed to get done with drivers business.
4. Then I enabled SATA AHCI mode to get my custom SSD running up to speed. Need to disable sleep mode as AHCI and Sleep mode do not work together. You don't need to enable AHCI mode if you don't have a SSD drive.
Done.
There you go! iMac 2011 running almost as fast (or faster) than 2012/2013 iMacs. It is also running latest Apple software and the drivers are up to date.
If still in doubt pm me.
If you want to install the easy way, download this custom bootcamp 6 package i created for 2011 imacs and run setup.exe. It will throw some errors that few driver packages were not found but will continue with the setup regardless. Reboot. Much less bloated system. But with this method, you will get few driver installed entries in "program and features" uninstallation window.
Click here to download for 2011 iMac only.