the pci-e standard voltage variables are quite outdated really, in terms of what can be supplied and what the standards claim is the max.
Still 300w is quite a bump, and im not convinced gpu mfrs would utilize it... however modded bioses and liquid cooled cards would probably benifit a BIT. but id still imagine you would hit a thermal ceiling well before you reached the maximum power output of 300w.
I also dont imagine boards would become Exessivly expensive. in my mind its not much of a change few extra mosfets some extra caps, few resisters and diodes. possibly bulk up a few traces and increase isolation at a few key points. Initial design may be a bit of a head ache, but seems they have already covered that part, so the constituent parts after that really don't add up to much. perhaps 5% extra cost when you also factor in development costs.. But even then a couple of generations in dev costs will have been covered any way, and a few components extra really dont add up to anything when you have boxes full of them..
just seems a rather strange thing to target, as i cant see how it benifits hardware manufacturers, the only immediate benifit i can see is media center builds or things akin to a steam box. where you could have a card with 0 power connections attached and just draw it directly from the board.
On the user side.. I guess you could chose to have 0 pci-e connectors and just use the slot for power to make a nicer looking case. or as i mentioned go all out with all the power you can throw at it and over clock