To be clear, you don't get this error when previewing at 100%?
What version of OSX 10 are you running?
10.10.4 but happens in 10.10.3 also, yep only at 200%. It used to be worse and you would get screen tearing and poor audio performance at 200%. So an improvement but still means 2015 is unusable on these iMacs,
Playback also drops out of realtime at 200%. This all works fine in 2014, apart from the RAM issues.
That's an annoying issue.
What are your preference settings in the previews section set as? Specifically, what are the Zoom Quality and color Management Quality set to?
What happens if you change them?
Also in the preferences, on the display section, do you have hardware accelerate panels turned on or off?
What happens if you change that?