So besides noise & heat, does it really pay to underclock vs. stock on the SP20?  I tried to run the numbers allowing a total of 4200 watts at the units (not wall).  I used 700w for underclock at 1200gh/s and 1200w for 1600gh/s.  My calc shows about $697 higher price for 6 underclocked units vs. 4 stock units.  The underclocked higher hashrate would take over 600 days to return the higher price of underclocking.

Am I missing something?