You can change everything in the BIOS. I don't know what you refer to as a multiplier, there should be an option to just change the speed to 1600. Make sure you are running the correct voltage (probably 1.5v) as lower than rated will make your system unstable. I notice you OCed your i5 already, well RAM works the same way. The lower the latency, and the higher the speed, the more voltage is needed, however, if its rated for that speed and latency you should just be able to change the settings. Look for Intel X.M.P memory profile, that should change the settings from the 1333 CL9 default to whatever the RAM is suppose to run.