Clive,
Agreed...
Any narrow band ITF filter will be susceptible to off axis wavelength drop.
I assume all/ most of the extreme narrow band filters we're talking about - <5nm FWHM bandwidth will "approximate" to the Gaussian wings...
http://astrodon.com/uploads/3/4/9/0/...ii3latestt.jpg
That being the case, there will be a reduction in the FWHM when stacked...