You should swap in your 16mb cache drive for your boot drive. The 16mb cache is beneficial because OS X will have more 'room' to store it's commonly used files. Thus, making your experience faster. Make sense?
By the way, the whole SATA 1.5 or SATA 3 is a marketing gimmick, currently, you can not even saturate or even come close to saturating a 1.5 drive.