When comparing the SSD 850 Pro to the 840 Pro using the Atto Disk Benchmark read data we see that the 850 Pro is faster when working with 1K, 2K, 8K and 32K data, while the 128K performance was much the same.
The only SSDs to really challenge the SSD 850 Pro was the SanDisk Extreme II and surprisingly the Crucial MX100.