Actually, most times you'll find that luminance subs are still filtered to block IR. A luminance filter simply lets in the visual spectrum. Shown in the attachment below is the Luminance bandpass of a
Custom Scientific Clear (luminance) filter. Keep in mind that most CCD cameras are sensitive on either side of the visual spectrum. There is nothing wrong with unfiltered exposures, in fact it is good for galaxy luminance work considering they emit across the spectrum, but typically you're only after the visual wavelengths anyway. IR blocking is critical for RGB work to reach correct colour balance, not so much luminance work. Astronomik manufacture a
multi coated clear filter (MC), that errrm... does nothing. weird.
I personally have not performed any background ADU measurements to determine the performance differences between unfiltered and luminance filtered exposures of the same duration. Nor can I comment of using specific LPR or Nebula filters. Perhaps someone else has done tests...