Come on now, I've been told on many occasions that 300W is just an arbitrary number. Now you're telling that the "powers that be" are actually knowledgeable. Who would have thunk it? /sarc
Seriously though, thanks for stating what should be obvious to everyone. The PCI-SIG is made up by professionals from all fields in the industry. They work out specs that are the most beneficial to everyone in the industry. They are a compromise, as these things have to be to take everyone into account, but it doesn't mean that the people on one side of the isle should just ignore the regulations they've all worked out.
http://www.anandtech.com/show/4209/amds-radeon-hd-6990-the-new-single-card-king/5
Not that I am at all surprised by this, it is now just one more example of what has happened time and again in this industry.
No different than the CPU heatsink spec weight limit violations for the ATX or the Vdimm spec voltage limits for DDR2.
And FWIW you completely mistook what I was trying to communicate regarding the word arbitrary. From an engineering standpoint the number "300W" is arbitrary, the arbitrariness is self-evident in the lack of significant figures associated with the number. If it were an actual engineering constraint then it would be something like 301.135W, a number with precision in its specification that would belay the data-driven nature by which the number was generated.
As it stands, 300W is no more significant than 301W or 299W, the last digit is insignificant (mathematically and scientifically, hence the number is arbitrary with respect to any other number of equivalent significant digits).
You've probably heard this phrased as "within rounding" or some such. But in engineering the term has significance. 300W is as arbitrary as 299W and as 301W.
Nevertheless the bigger picture is just as was described and discussed earlier in this thread. Spec limits are born from real limitations and concerns, be they economic or engineering limitations. Resolve those limitations and you can make a product that violates the spec but addresses the root of the limitations captured by the spec in the first place.
This happens all the time in the world around you, it is reality, scream at if you like but until you are an engineer who has worked in the field and made products that see the light of market you will probably never fully understand how the system works or why it works the way it does.
From my perspective all I can do is try and convey a bit of this reality to you, and I'll never be good at it as most engineers have the communication skills of a rock and I am no exception.