so, computerbase did some testing.
They showed that games will stutter heavily or freeze if you force them to use the slow memory.
1080p gamers are most likely not affected (yet). SLI and/or multi-monitor users will suffer from problems much most often.
Case in point:
http://www.computerbase.de/videos/2015-01/gtx-970-vs-gtx-980-2-monitore-far-cry-4/
FarCry 4 before and after (51s) connecting a second monitor. Second monitor is showing aero desktop.
http://youtu.be/MTYd9_fe4iI?t=57s
Talos principle long duration freezes
Cant argue with that.