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Nice review, and my experience with my Astrodon NB filters mostly matches yours.
However I decided to go for a 5nm Ha instead of the 3nm for 2 reasons.
The first is that while you're correct that the narrower OIII and SII filters do get the same signal at the relevant wavelength, with much less noise, this isn't necessarily true for the 3nm Ha filter. As I understand it a Ha filter of about 5nm and above is also passing NII light whereas the 3nm is only Ha. This will mean that when imaging objects that are relatively bright in NII, such as planetary nebulae, you would require longer Ha exposures to get an equivalent amount of signal, or you would also need to image through an NII filter and combine that in your processing. Either way your imaging time is increased, but of course only on objects that are bright in NII.
I've got no idea how much this would impact "in the real world" when imaging a nebula like M16 as I've never seen an image of it through an NII filter.
The other reason is that I've always found that my stars are much tighter in Ha images relative to OIII and especially SII images so I figured the slightly larger stars through a 5nm Ha vs a 3nm Ha filter will help me balance my magenta stars in my NB images.
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