MarkS, "x24 gpgpu (35 TFlops) ~0,84 Pflops" - 3 servers with 8 top videocards in each in 2016 is 0,84 Pflops ..
This isn't something that you have to wait for. Massive render farms exist with current technology that far exceed what you posted. Pixar and Weta Workshop have massive render farms with hundreds, if not thousands, of servers containing several GPUs each, all arranged in parallel. Even with this massive amount of computing, all of which is geared towards graphics, it still takes hours per frame to render.
In this case, it comes down to a software issue. Most programmers do not know how to write truly parallel code and GPU drivers do a fairly poor job at it. Because of this, the load is not balanced across the GPUs. 8 GPUs does not equal 8x performance. And there are still bandwidth issues at play.
You are falling for a common computing misconception. People think that throwing more processors and/or faster processors at a task will speed up the task exponentially. The truth is that it wouldn't even speed up the task linearly. It is similar to throwing money at world hunger, thinking that the more money you use the less starving people there will be. That is only true is all of the money gets to the starving people (it wont and never does) and if money is the sole reason for their lack of food (most often not). Point being, it misses the small and yet incredibly important technical details that hinder the plan from working.