“No Nikon camera moves 8 x 24 mpx per second to the buffer. That's 192 mpx/second. The D4 only moves 160-176 mpx/second”
Interestingly though – the 7D at 18 mega pixels has a file size of 25 MB (bigger file sizes than any of the 24 MP Nikons) and at 8 FPS it is moving 200 MB per second – and they were doing that in 2009.
I think that all of Nikons speed limits are self-imposed rather than technical limitations.
The D4 may be at or close to the mechanical and AF limits for frame rate.
I think 7 FPS is more likely for a D400 – the same speed as the D300s without a grip. That would require moving about 160 MB per second to the buffer. (RAW lossless compressed) 6 FPS is my bottom limit.