From my experience, it seems that overclocking the S2 just makes little sense.
Replacing the PSU (and paying an extra $350) + increasing load by 0.5amps just to get an extra 50 GH/s seems more risky than the value it will bring.
Would love to see what other people think.
Thats what I also concluded and stopped. Its just not worth the time, power, potential for failure or effort.
A 1200W psu can get 1050-1100GH from the unit. Clocking anything higher would require pencil modding of 40 resitors per S2 unit.
IMO - the S1 is still the king. about $1.15/GH when overclocked, and can underclock to ~160GH/300W without voltage modification, and probably as low as 110GH/120W if heavily underclocked during the summer months. Only issue is space requirements and the power/cooling requirements when running them at 2w/GH initially.