An 18mp sensor produces the same size file if it is a full frame sensor or if it is an APS-C sensor. Smaller pixels do not make smaller bits
Write time to your card is based on lots of factors, but the two main ones are processing delay and the slower of card speed and camera write speed.
Processing delay is not as simple as faster processor beats slower processor.... What is the algorithm? Does it take the same time for an iso100 image as an iso 51200 image with lots of processing to correct noise.... Does a 60D run a simple algorithm on a clean picture faster than a 1D X on a noisy picture??? This is why there is no answer to how fast can a digic 5+ process pixels.... The answer varies based on the algorithm used.
Write time is the slower of camera write speed and card speed. I don't know how fast the 1D X write speed is to the card, but you can bet that the published specs were with the fastest card available. Nice and simple...... And wrong! After the image is processed it is stored in high speed memory, which is then dumped to the card. The bottleneck occurs when the read/process/dump to high speed memory occurs faster than you can write to the card.
A 1D X has something like 34 times the processing power of a 60D and we all know how the mighty compact flash beats the tar out of the lowly SDXC cards, yet the 1D X is only able to take 12 frames per second compared to the 5.3 of the 60D....... There are no easy answers here.