write latency scatter?
As important as write speeds, is write latency. It's no good having high throughput if the drive hiccups occasionally.
Sandisk have a paper up about this at http://www.sandisk.com/assets/docs/performance_consistency_final_08_26_13.pdf
Several websites started looking at this seriously when intel pointed it out as part of their datacentre range release - and sure enough a lot of consumer-grade drives sometimes take tens or hundreds of milliseconds to complete a random write or read. This has a huge impact on overall performance.
A good example of the differences between drives is shown in the charts at http://www.storagereview.com/intel_ssd_dc_s3700_series_enterprise_ssd_review
It'd be extremely interesting to see the scatter plots for the 850pro and the Extreme Pro - this is the kind of thing which makes the difference between "fast" and "not as good as it should be"