Bad Read/Write speeds on an SSD? 10.10.5

I have a 500gb Samsung Evo 850 SSD, and I'm only getting half the read/write speeds it advertises. BlackMagic says 256/270 MB/s read/write on it. my system is a core i5-3550 Ivy Bridge processor, 16GB DDR3-1600mhz ram, and a GTX 770 on a Biostar TZ77-B motherboard. TRIM is enabled.

Black Magic screenshot: http://i.imgur.com/Mx6RMWW.png

System Info: http://i.imgur.com/lWjENI7.png

System Profiler: http://i.imgur.com/cFgKM9j.png

submitted by PrincessZoey89
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What is missing? Please comment to help!

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