Yes, partitioning Iraq would be in the US national interests, because it would prevent another large conflict with Iraq. Iraq as it is partitioned would be too small to organize enough to cause large problems for the United States. The United States could focus its defense efforts on other nations causing problems, like Iran.
I think that partitioning Iraq would not be in the U.S. national interest. There is a possibility that more violence and a surge in terrorist group's activities would result after partitioning Iraq. This would lead the United States to send more soldiers to Iraq and spend a lot more money. That is not ideal for us financially, nationally, or otherwise.