Ok these are the results of my tests on my sab triple unix ( i believe the box runs 450mhz without the hdmu image). The times measured are from turning the box on with the switch on the back untill getting an image on bbc1 (fta)

540 (default)-> 1min 19sec
630 ---------> 1min 15sec
710 ---------> 1min 14sec
775 ---------> 1min 13sec
800----------> 1min 11sec

So there is an 8 second difference between no overclock and full. Not an awfull lot