External display for gaming on 2011 iMac

[Mightee]

I've been playing Diablo III on my my 21.5" 2011 iMac (i5, 6770M 512), and while I really enjoy the graphics performance, my screen size is a bit small for my liking. If I hook up a second display (not a Thunderbolt) and use it as my primary monitor, what kind of performance hit can I expect from the card?

Thanks!

 

[JaHull]

If you are going to up the resolution to 2560x1440 or higher you can expect quite a big performance hit on my iMac which is 2 years older than yours with a 4850m i go from 37 down to 25 FPS when switching from 1920x1080 to 2560x1440 and when comparing my graphics card to yours on notebookcheck.net with Starcraft 2 your graphics card is around 10FPS lower on High or Ultra settings, which is what I use on Diablo 3.

On the offhand of you just getting a larger display with the same resolution as your current display or a larger display but are willing to play Diablo 3 at not native resolution you should be fine.
I have a second display connected to my iMac and have never noticed any difference in FPS when it is connected and when it is not.