The TX series has been manufactured for Corsair by Channel Well Technologies (CWT), unlike TX v2 (check the model number) which is being made by Seasonic.
This is a common problem with PSU brands: the same brand and wattage doesn't mean the internals are the same or even made by the same manufacturer.
The TX seres uses DC-to-DC conversion and should give you 620 Watts at the +12 rail.
Of course, part of that +12 power is being consumed by the CPU, fans, and drives. The other rails (3.3 and 5v, often called minor rails) are of course also being utilized by the rig, lowering the amount of current you can draw from +12 lines.
I wasn't able to fine overload tests for this exact model but is bigger brother (tx750) has been successfully overloaded by 20% (a total of 900 Watts on the DC side) at the cost of increased power noise and ripple.
You wouldn't have an electricity usage monitor (a popular brand is Kill-A-Watt) lying around?
IMO every serious miner should have on for determining and fine tuning the power consumption of their rigs.
Maybe grab some knock-off from e-bay?
Or, you could give me the details of your rig and we'll try to assess the idle power consumption and thus - how much juice this baby can pump into the cards.
Links (if you're interested in the technicalities):
This is how a proper PSU review is made: http://www.hardwaresecrets.com/article/Corsair-TX750W-Power-Supply-Review/505/1 http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=254 http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=230 http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=73
Unlike this: http://extreme.outervision.com/articles/psu/corsair/tx650/corsair_tx_650_1.jsp