The other side to the upgrade to the Falcon 9 is chilling down the fuel more than normal so that it shrinks and they can fit more of it in the rocket's tanks.
Do you have any reference for that? It seems to me that further cooling already liquified fuel doesn't give you any noticeable difference in volume.
r/SpaceX has a lot of good info. Check out the "What are the differences between the Falcon 9 v1.1 and the upgraded Falcon version popularly called the v1.2?" FAQ here: https://www.reddit.com/r/SpaceX/wiki/faq/falcon
Why not? According to a random page I found on the internet, kerosene has a coefficient of thermal expansion of 0.00099 per Kelvin. If you can cool it by 50 degrees then you'll be able to store about 5% more fuel in the same volume of tank, which is a nice little upgrade.