Actually it was answered in your thread on the TWC. Here:

To specifically increase siege equipment movement you can add the following lines to descr_campaign_db.xml:

<siege_movement_points_modifier float="1.00"/> <!-- movement points modifier for siege units -->

You add them in the lowest section of the file, the "misc" section. I think you need to run the game with the kingdoms.exe to be able to run it though.
http://www.twcenter.net/forums/showthread.php?t=119655