I have a MacBook Pro 15" mid-2012 (i7 2.6GHz, 16GB RAM, GT 650M 1GB). I installed OWC Mercury EXTREME Pro 6G SSD in the hard drive bay. I ran Blackmagic Disk Speed Test and got only 220 MB/s write speed and 460 MB/s read. OWC's website says they have 527 MB/s write and 559 MB/s read. So I am far far away from this. Does anyone have any idea what the problem could be?
Check to see what your Serial-ATA link speed is. You can find this in system profiler.
It will be something similar to this.
If your speed isn't 6 Gigabit but is 3 Gigabit then that would be why you aren't getting the speeds you'd expect.