To answer the OP, I'm pretty sure the limit is in the mechanical shutter, however, like Mt. Spokane mentioned, it's likely that the capabilities of the processors and the buffer are matched to the limitations of the shutter mechanism. It makes no sense to over-engineer something if it can never be used. If they could get 14FPS with the shutter moving, they wouldn't have needed it locked, and it would have been better for them if they could have allowed focusing at 14FPS. Saying JPEG only is not dependent on whether the shutter moves, either. Also, it should have been (relatively) easy to slap more buffer RAM on the board if it were needed to handle more data.
Card speed is definitely a limiting factor, though, but more on the continuous shooting side. If you put 1GB of buffer memory into the camera, but it takes 8 seconds to clear it (and that's with Lexar 1000x cards), once you fill that buffer you're going to be waiting if you want to burst again. They might be able to fit 4GB on there, but then you need to wait half a minute to write the whole buffer out. I think the processors can handle a bit more than the 150MB/s of data the card can, especially if you're shooting clean images (ISO 100). The more noise and the more cleaning functions you turn on, the more you'll tax the logic. And they could always go for more chips if they can find the room. Think quad Digic 5+.
As to the later posts about file sizes, consider this: a 7D and a 1DX are really close to the same MP (same written image size of 5184x3456). However, a 1DX takes FAR cleaner images under almost all circumstances. Cleaner images = better compression. The idea is that a cleaner image is likely to have more pixels that have the same value, so the compression algorithm can more efficiently store that. If every pixel is a different color (because it actually is or because of noise changing the output just slightly), compression falls apart. I just took two pure white images (.6" ISO100 no lens, shot at a lit wall) from a 7D and a 1DX. Both show they're completely blown out in LR. Results: 14,498,781 bytes for the 1DX and 14,223,956 for the 7D. I'd guess that 200KB is mostly extra metadata.