Just a theory.. How many drives do you have? I wonder if it cuts the rate in half when 2 drive are connected to the same interface. In other words, does it drop to sata/150 when you use both Port 0.0 (SATAII_1) and Port 0.1 (SATAII_2), or Port 1.0 (SATAII_3) and Port 1.1 (SATAII_4)? Maybe it will hold at sata/300 if you use 0.0 (SATAII_1) and 1.0 (SATAaII_3), but leave Port 0.1 (SATAII_2) and Port 1.1 (SATAII_4) empty?