Interesting.. this is some number LACT came up with, or does it seem plausible due to other stuff, like real measurement of the cards power consumption or at least some benchmark scores.
As a softworker, i now one thing: never trust software. Not even your own.
Of course, hardmods also work. On my GeForce i simply soldered another shunt resistor in parallel, and now i have ~380W limit instead of 305W. Works also on Radeons, since the power signals from the controllers are somewhat tangled to the measurement on the shunts. Curent limit, though, not so much, but it is recommended only to go beyond this limit with proper water cooling.
So, 400W or something is totally doable. You would lose warranty, if you care about that.