most likely the thermal paste that comes pre-applied to the cooler is not going to be top of the line stuff. That being said what cooler are you talkinga about? Applying it CORRECTLY yourself would cause the cooler to perform a little better but at most only a few degrees C under load. To most people this is not worth the hassle especially if you do not have good thermal paste laying around already. If it were me I would just stick with what came on there unless you are looking for the absolute lowest temps to get the absolute most out of your OC