One dude on techpowerup forum did some black magic and created small app for doing the background work with drivers and I did some refactoring of that. His post is here:
https://www.techpowerup.com/forums/threads/sli-with-different-cards.158907/page-123#post-3996362
My fork:
https://github.com/MaciejGorczyca/DifferentSLIAutoLoader/releases
Basically benefit over manual work is that all you need to do is grab your nvlddmkm.sys file, modify it in HexEditor, go to safe mode, replace the file, boot back into the normal mode and use install script then reboot.