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Right now the high-res cameras like the D800 have painfully slow framing rates.
I'm not sure i'd call 4 fps "painfully slow". Still, my back of the envelope calculations put the data readout at about 240MB/sec off the sensor at 4fps so you would need a nippy card to keep up with that, assuming that the onboard processor is capable of running through all that data in real time.
The question remains as to how many folk actually want 30-50 megapixels at 8-10fps. Thats a hell of a lot of data - a D800 at 8fps would give you 480MB/sec, and would fill a 32Gb card in around 68 secs if it could keep up. Summed over a day's shooting i'm guessing you would generate an enormous amount of data, and whilst I know hard disks are "cheap", they aren't that cheap. Do action shooters really want this kind of resolution, and the accompanying data storage and image processing overhead?