The change the sensitivity (and speed, etc.) permanently, you can create an udev rule.
E.g. create a file 10-trackpoint.rules in /etc/udev/rules.d/ with the following contents:
SUBSYSTEM=="serio", DRIVERS=="psmouse", WAIT_FOR="/sys/devices/platform/i8042/serio1/serio2/sensitivity", ATTR{sensitivity}="220", ATTR{speed}="190", ATTR{inertia}="6", ATTR{press_to_select}="0"
This method was tested on Ubuntu 14.04 up to 15.04.
For further explanations check on the Archwiki and Thinkwiki and look for "sensitivity".
Update for Ubuntu 15.10
The above rule does not trigger properly on Ubuntu 15.10. Instead use:
ACTION=="add", SUBSYSTEM=="input", ATTR{name}=="TPPS/2 IBM TrackPoint", ATTR{device/sensitivity}="220", ATTR{device/speed}="190", ATTR{device/inertia}="6", ATTR{device/press_to_select}="0"