So Gamer Nexus site that discusses computers and all things related released a nice video sowing the upgrading from single GPU to 4 GPU setup. And even a CPU comparison.
Puts things into perspective. Especially where one could spend more money on to get best results in rendering. (GPUs 
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So that brings up a question, where does the new Intel X299 and AMD’s Ryzen Threadripper bring value to rendering? Particle simulation? Can’t think of anything else at the moment