sirs: currently at edwards and associates this afternoon. checked on the latest serial no. 412 BHT FMS, the nightsun limitations is for a particular bell kit and does not apply to the aircraft in general.
the flight manual supplement only applies if that particular kit no. is installed on the airframe, therefore another stc is not "over riding" the BHT manual
with regards to placement of nightsun, it would be the same for flir / winch / floats etc, they would all have to be tested to determine the effects on stability.
dr