I don't know where you live, so I don't know which regional store would be relevant. I'm in the EU, so Custom Cable is one alternative there. As for the US I'm not too familiar with the sites that would stock it.
I'm a huge fan of the K 701, so it feels inevitable that I'll get the K 812. I found both the Beyerdynamic T1 and Audeze LCD-2 to be worse than the HD 800, so I hope the K 812 can keep up with the HD 800 a bit better. I assume finding out how good they really are will require more than just the quick impressions we're getting now. Who knows, maybe I'll be an early adopter. At least they don't seem to be a disaster like the Ultrasone Edition 10.
Yes, the P7 are quite a bit bigger than the P5. The P5 are the right combination of practical size, comfort and isolation which is rare. The P7 strike me more as headphones you'd use at home, on walks or commuting. For longer travels and flights something more compact might be more ideal, though the P7 would probably do just fine there as well. I've ended up using my P7 at home the majority of the time instead of my HD 800, and outside I'm using the Kef M500.
A lot of the extra cost comes from the fact that they're packing in so many features this time and not skimping on quality. Stepped attenuator volume controls are much more expensive than the standard ones most use because of the number of parts it requires. The power supply has also been upgraded, as is the dac chips themselves, and there's more inputs and outputs. Across the board there's more of everything, and it's of higher quality. It all adds up. Will it sound...