For example, Samsung 830 or Intel's 520 ssd has a read speed of somewhere 550MB/s. Sata 3 has a maximum bandwidth of 750MB/s (6gbps / 8). From my understanding the 750MB/s max speed is theoretical and the real-world speed is 2/3 to 1/2 of the theoretical max bandwidth.
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Mathematically you are correct, 6Gb/s = 750MB/s; but when referring to SATA ports you are incorrect.
The maximum theoretical speed of a SATA 3 (6Gb/s) port is 600MB/s. The difference is due to SATA protocol overhead.
You're pretty much on target.
Actual performance during real world use will vary quite a bit. It depends on what you are doing with your pc.
Also, not all SATA 3.0 controllers perform the same. Some are closer to their theoretical maximums, and some are lower. Either way it is borderline.