It is sooo worth it. I've got people driving behind me now swirving to avoid hitting me cause the braking power has been improved soo much.
I got the 330 front calipers and carriers for $200 from a member sponsor here (eurostyle) and bought a good set of new slotted rotors and hawk pads for the front and rear - needed to change the brakes anyways as I had two faulty calipers on the front, so I said why not upgrade instead of spending close to the same money to get the same stuff. Overall I dropped $700 in total for the 330 calipers, new slotted 330 front rotors and pads, plus new rear 325 slotted rotors and pads from the states (with shipping).
Since you said you have slotted rotors already, get the 330ci front calipers and carriers, the 330ci front rotors (slotted) and Hawk Pads and you are set. It is a little different since it takes some time to get used to having to warm up the Hawk Pads, but once they bite into those nice large rotors, watch out ... you'll have some serious stopping power at your feet.
First day out I had the guy behind me have to swerve into the shoulder on the left lane on the 401 cause he didn't expect me to stop that fast.