Liquid is a much better carrier of heat than air, so a properly used liquid cooling system is very efficient. As far as percent goes, I’m not sure but I’d be willing to say about 35-50% more efficient. For this reason, many overclockers use liquid cooling systems.
If you want to overclock, I say liquid cooling is a good option. There are plenty of units available if you just google them. They are worth the trouble if you are taxing your CPU to the point where more than a conventional fan is necessary, but keep in mind, they have improved fan and heat sink designs available too.
Cooling systems don’t really speed up processors…they maintain the life of processors by keeping them at a temperature at which they can safely and efficiently operate. Processors can be sped up by overclocking them, at which point cooling is essential for any performance at all.