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My conclusion was based on comparison between shots taken (by others) with and without filter. When tweaking images to achieve the same (or similar) colour balance, the noise with filter appeared to be actually higher - so I simply stopped thinking about it as a solution.
Also, sodium lamps have quite a wide spectrum (they are mostly high-pressure models) so filter is not really effective because it cuts off too much of wanted light from celestial objects.
The only real and significant improvement in light polluted areas is possible with narrow-band filters - but I haven't done this (yet).
Other way is simply to do what's possible to do - stellar photometry, for example - with high F-numbers, LP is not such an issue.
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