Hi, I'd like to run another 4k monitor next to the 5k iMac. Will the imac keep up? Or will I deal with lagging/beach balls? I'm not doing any video or gaming, just need the screen real estate to code. Rather use the 5k iMac since I get an IPS 5k screen plus a computer, for the same cost of a Mac Pro, without the screens... Would upgrading the GPU make any difference in my case? Thanks!