That iMac is very much limited in resources. You can add memory, but you can't upgrade the GPU or the processor and I doubt you would be happy with the performance with only 512MB VRAM. Refer to the new Configuration Guide for guidance (page 13):
"The 27” late 2014 Retina 5K iMac with 4GB GPU RAM provides a flexible and versatile desktop workhorse for real time editing and grading material up to and including HD video. There are no special setup requirements for the 27” late 2014 Retina 5K iMac. The late 2013 iMac with the 4GB GPU RAM model is also suitable.
With both models UHD timelines are practical with some performance limitations especially when using optical flow speed changes, temporal processing and noise reduction.
The new 27” mid 2015 Retina 5K iMac with 2GB GPU RAM and many of the older model iMacs with 2GB of GPU memory will reach these GPU limits at lower resolutions and with fewer editing tracks or color grading nodes. Very limited performance is possible on the 2010 and 2011 top of the line iMac’s and these systems will not be suitable for any temporal processing, NR or images greater than HD resolution."
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