SATA6 is not the same on every motherboard (80% of them are poorly implemented like only using 1 PCI-E link for the SATA 6 Raid chip or an AMD native sata 6 ) have you tried it in an Native intel or amd Sata 3 port
What software did you test as bigger drives can show worse performance if you do not limit the test to less then 50%
I would only be interested in the first 40-50% of the disk performance as after that it really does drop off quite fast, why I norm partition the disk 30/70 (short stroking) so OS and programs are forced into the best part of the disk (all programs or games should be installed onto the c: drive, the rest as storage for none active files like video, music so on), that is asumeing your using it as an boot drive how i would do it if i had an HDD as an boot drive (SSD owner)
Sata 6 for HDDs bit of an farse, no way an Currant HDD can do more then 150MB/s never mined 550MB/s that SATA 6 offers, SSD is the only thing Sata 6 is useful for (Ignore Burst speed as that not important at all, unless the HDD has 500mb-1gb of cache on it)