JC,
I'm still a ilttle unclear on what you want to do.
The rejection of external, input common-mode noise should be covered by the device's CMRR.
A differential noise source will be subject to the device's nominal differential gain.
Attached to this thread is a modified version of the reference circuit you can find at the device product page:
http://www.ti.com/product/ina111/sboc137
The existing circuit is set up for diff gain. A noise source applied to the same connections as VIN will result in diff noise out. A second circuit has been added for input common-mode. Running an AC sim will give you the corrresponding diff & common-mode gains.
Please ley me know if this helps, or if you have any other questions.
Regards,
John