Performance does not scale linearly with clock speed at all. (try 500mhz and 800mhz and you will see)
Of course it does. Unless perhaps if you are using SDK 2.6.
A simply test shows it does not.
870Mhz 397Mhash/s 870/397=2.191
800Mhz 359Mhash/s 800/359=2.222
500Mhz 202Mhash/s 500/202=2.475
In theory it can't be linear, same reason why cpu does not scale linearly with core speed.
Don't spread false information