No matter how cool your system is, it will not run faster. In a slight sense it will. By this, i mean your computer wont have to stress under heat while dealing with applications. Basically, your system will run at the rate its suppost to. So it dont increase speed.
I dont know who told you your system will run slower at low temps unless you OC. Thats not true at all. Having a low temp, is having a low temp. Now if your system gets to cold, with OC or not. It can result in physical damage. But that kind of temp, is beyond anything we can get too. Im talking about -200 degrees celcius.
Finished With TF.