In my experience, I've found that these cooling mats don't seem to really make a difference. If you have the laptop on a flat hard surface and it's getting unobstructed airflow into it's vents already, a cooling pad probably won't improve the situation at all. Where cooling pads help are when you are using it on a bed perhaps, or if it's directly on your lap and a vent might be resting on your clothing. Or you just don't want a hot laptop directly on your lap.
As for taking apart the laptop and replacing the thermal paste. This could be a bad idea. Some heatsinks may not use paste, but a thick thermal pad. If you remove that pad and use paste, the heatsink no longer makes full contact with the CPU and the situation is now worse. Then you'll need to find the correct thickness of thermal pad online and it's not easy. This was a lesson learned the hard way for me.