This is exactly my question im looking for. I want to edit 4K material in a HD Timeline with lots of grading and plugins but with my iMac Late 2009 this is somehow a horror trip...even with Proxies.
I wanted to get a new iMac in the last weeks as i heard there´s something coming up and i really don´t know if this 5K iMac will fit my needs. I´m really, really unsure about that graphics card. because it seems only slightly faster it has to render 4 Times the pixels as the Nvidia in the "Old" iMacs. I think about getting the "older" iMac maxed out with the NVIDIA GeForce GTX 780M 4 GB GDDR5 and 32GB of Ram with a 1TB SSD would Top the 5K Version, isn´t it ?