And then the restrictions on T/O and descent wouldn't be as much as 2300 kgs and 1400 kgs respectively I'd guess..
One technician told me rather hastily that if one emptied the CTR tanks on ground one needed several tonnes of fuel to be refilled in the CTR tanks again to get them running afterwards. Why this isn't the case in air I have no idea about.
That's why I'm asking. I was told it may have been listed in the previous FCOM, prior to Boeing taking away a lot of information, but that was before I started flying the 737.