The voltage drop can be as much as would occur in 2 feet of standard wire. Worse yet, modular plugs utilize delicate pins that can easily loosen, corrode, and burn, creating the potential for a major system failure.
Don't read too much into that, it is not for specific brands,on the Hiper they use gold plated connections,plus aviation type screw on connectors that won't come off unless you unscrew them on purpose,pins are thick not fragile,as for the Hiper PSU, it is very good from my limit own experience, and 98 percentage of reviews back me up.
Over at OCuk forum in the UK they rate them too,also if you go to DFIStreet.com they are using them for DFI boards,one of the recommended PSUs.
so you have:
1- no efficiency rating
2- no clue about what temps the psu was tested at
3- power reduction due to modular plugs
4- probably inflated total wattage claim
That means nothing just your speculation.
Btw the manual gives these specs for what it is worth.
Output load regulation:
Max power shall be less then 630w.
Total power of +5v and 3.3v shall be less then 240w
Total power of +12v1 and 12v shall be less then 360w
Total power of +5v,+3.3v,+12v shall be less then 560w
Efficiency of 70% MINIMUM when measured under full load condition.