The S5 has some serious overclocking potential! 1.3TH seems to not be a problem and 1.4TH may be possible. (and its likely that efficiency will not change significantly)
This could be the info that changes my mind to go ahead and pull the trigger on a couple of these S5s to replace my two S3s.
if you have cooler air for the intake and can handle the noise, then yeah theres definitely some room to overclock. 400MHz seems to be no sweat with a pair of 650W PSUs (I was achieving 387.5MHz on a single 650W gold (612W limit on 12v) without tripping the supply, but cables were warm so i brought in a 2nd PSU to go further