You need to change them when they start squeeking. There's this little metal tab that will hit the rotor and start squeeking when they're worn down almost all the way. You can take off your wheel and try to see if it's almost time. Look for that metal tab, or just try to see if the brake pad material is worn down to about 1mm - 2mm.
I have the AEM/Nissin pads and I can tell a difference. The only thing is that you have to warm them up a little bit before they'll really bite. This doesn't mean that they bite less when they're cold than the stock brakes do though. They're slightly better when they're cold, but really nice when they're warmed up a little. You can feel them bite in.
The purpose of new brake pads isn't to stop sooner though. It's to be able to brake longer and harder without having the brake pads lose their effectiveness. Same thing goes for new cross-drilled/slotted rotors. Their goal is to keep your brake pads from glazing over and to keep them cool. You'll only be able to stop faster when you upgrade your tires, since you'll have more friction against the ground to work with. | 