"I would very much like to hear which particular lenses for DSLRs or
35mm SLRs produce a 2 stop reduction in illumination from that
expected at any given aperture."
2 stops is an awful lot, although I suppose some ancient zoom lenses
with lousy coatings might be that bad. One of the most complex zooms
I'm personally familiar with has about 40 elements, but nevertheless
suffers less than a 1-stop illumination reduction.