The 850evo m.2 is not any faster then its sata brother. Its still just a sata drive using the sata lines on the m.2 port.
The 950 pro will definitely be bottlenecked by the x2 pcie2 lanes but would still be nearly twice as fast as the 850. (900-950Mb/s vs 500-540mb/s)
Its up to you if the 950 is worth it at twice the cost and getting less than half its max speed. (2200+mb/s)