Does heat affect the speed of a CPU?

[AmigaMance]

I came across to this thead here: http://www.amiga.org/forums/showthread.php?t=1299                                    
In which a user claims that he got a big speed increase from his ppc CPU, by replacing the heatsink with a bigger one.. :-)                                                  
Is this claim, valid? I know that heat affects the stability of a CPU and has an extremely small impact in its speed, but this is the fist time that i read something like this.

[AmigaMance]

 From my personal experience, i agree with Ilwrath and koaftder. I haven't notice any corelation between temperature and CPU speed. That doesn't mean that Framiga is wrong, maybe he has experienced something that we haven't.

@Framiga
 Are this changes measurable from a benchmarking utility, like SysSpeed for example?

@Chunder
 No, as Ilwrath said, i don't think that this is the case with PPCs.