I am going to answer my own question, in case someone wants to know or searches.
Just got the 5k iMac, maxed. 32GB / 4Ghz / 295.
I use an additional Dell Ultrasharp U2412M with an resolution of 1920x1200. I booted my Windows 10 install from an external SSD. Installed the bootcamp AMD graphics driver. Then right clicked on the desktop and selected to show desktop only on display 2.
This deactivated the iMacs display and only let the Dell display be active. Meaning I could play some games, in this case Metro Last Light Redux in stable 60+ fps with high graphic settings and 1920x1200 res. I didn't have time do to any detailed benchmarking but it was wery smooth and the fan didn't make that much noise but it did speed up in some high intensive scenes.
It doesn't work in OS X though, or I haven't figured out how to do it.