Hi Cris,
The proximity detection is implemented by a RC circuit, the capacitance measurement depends on the charging time of the RC circuit formed by the electrode and the pull-up resistor. In order to have a different sensitivity you have to change this parameter. You can find this information in the section 5.2.5 Sensitivity and range (http://cache.freescale.com/files/sensors/doc/user_guide/TSSUG.pdf?&Parent_nodeId=&Parent_pageType=).
If you are using the Freescale's Touch Sensing Software you can change this parameter with System_Sensitivity_Register. Refer to the section 3.8.2 in the same document.
I hope this information can help you.
Regards,
Adrian