In case it's of any interest.. recently built an installation using an iMac Pro feeding 3 x 4K video walls made up of 3x3 55in panels running as one continuous screen at 11520x2160.
Works like a dream.. had 9 layers of varying elements from 1280 to 4k without even a blip to the machine..
Expensive but seems to have lots a room to push the hardware..
S