I edit mostly 2K and already have Samsung 850 Pro SATA SSD´s in the computer. There are many factors that comes into play so a straight yes or no answer is hard to give. There days there are often something else in the system that is the bottle neck, such as CPU, RAM, media used, application used rather than the HDD. (Not always though...)
I did a test with 4K media, ProRes, rendered out to the CineForm codec in Adobe Premiere Pro CC2015 (9.2). No effects on the video clips. Total length of timeline in Premiere Pro is 210 seconds.
Scratch media on Intel 750 1.2 TB PCIe SSD:
Export to Samsung 850 Pro SSD: 196 seconds
Export to Intel 750: 194 seconds
Scratch media on Samsung 850 Pro SSD:
Export to Samsung 850 Pro SSD: 261 seconds
Export to Intel 750: 262 seconds
My processor is Intel E5-1650v3, 32GB RAM and what i noticed was that on my system the CPU is the bottleneck and it pegged at about 90% during renders while the HDD´s peaked at around 20-30% all the time.
The main reason i had for buying the Intel 750 PCIe is the fact that there were no free SATA ports aviable in the computer, so a PCIe SSD was the solution for me to get more precious HDD space. But if i had have more free SATA ports i would have bought the Intel 750 PCIe SSD anyway since the extra speed is nice to have.
