@@lm2k: As you have guessed, those current figures are just the max current limit for the respective rails. To get an ide about actual amps supplied, look for total wattage delivered on 12v rail and divide by 12. Say suppose its say total powered delivered on 12V is 300Watts...That means 300/12=25A
Now this 25 Amps can be divided like 10A on first rail and 15A on second rail.And that is why a single rail works better when rail current is low per rail...you would not want power hungry components on a single rail as the first rail tradiotionnaly is kept for CPU section
Regarding operating in lower voltages, i have personally used my gs600 at voltages 140-150V
, though certainly not advised.It may be able to handle but would definitely stress out your caps and voltage regulating circuit.