Let me revive this thread by adding some additional observation from my side.
First, Asus G551JW will throttle its GTX 960M, if temperature gets to 74C. That may result in around 5-20% performance loss.
There is official BIOS update that increases this limit to 78C, and unofficial beta driver that increases it to 82C.
All info on this thread:
https://rog.asus.com/forum/showthread.php?64295-G551JW-GPU-throttle-at-74-degree-celsius
Next, I did some utilization tests in the Novigrad central square during the day (running around).
I'm using Intel i7-4720HQ, quad core with HT, with 2.6GHz and 3.4GHz Turbo Boost.
I was testing downgrading CPU frequency (through power options) and reducing number of cores available (through processor affinity for witcher3.exe), and how it affects GPU utilization (used GPU-Z to test that).
8 cores, 0.8GHz - around 45-60% GPU utilization
8 cores, 1.0GHz - around 60-85% GPU utilization
8 cores, 1.1GHz - around 80-90% GPU utilization
8 cores, 1.2GHz - around 85-99% GPU utilization
8 cores, 1.3-2.6GHz - around 95-99% GPU utilization (more stable the higher the clock)
4 cores, 0.8GHz - around 35-45% GPU utilization, has sound shuttering
4 cores, 1.1GHz - around 45-55% GPU utilization, no sound shuttering anymore
4 cores, 1.3GHz - around 45-65% GPU utilization
4 cores, 1.6GHz - around 60-75% GPU utilization
4 cores, 1.9GHz - around 70-85% GPU utilization
4 cores, 2.1GHz - around 80-95% GPU utilization
4 cores, 2.4GHz - around 85-99% GPU utilization
4 cores, 2.6GHz - around 90-99% GPU utilization
As you can see, W3 uses multi-threading very well, and there is noticeable positive impact on lower clock with more cores.
Still, to be able to make use of 960M without performance drop in areas like Novigrad, you do need to have at least i5 with high CPU clock.
One more thing...
In my case disabling turbo boost has helped in performance (just put 99% as maximum processor state in power options).
This is because game would throttle GPU after reaching 82C, and that is much more likely with CPU trying to push 3.4GHz all the time. With this disabled, throttling would less likely happen and have lower effect.