Well, I had a chance to do a quick test tonight. I actually took shots with all four of my lenses but am only posting the results with the DA 35mm/f2.4 and the DA L 18-55 kit lens because they are the only two I could compare at 35mm.
Personally, I have come to the conclusion that the phenomenon of less noise with certain lenses was all in my head!
Now, it may be that shooting in bright sunlight would produce a different result. However, photographing my spice/cereal cabinet inside under tungsten light seems to indicate that there really isn't any difference in noise level at either ISO 1600 or ISO 2500. If anything, the kit lens might actually be slightly better in that regard to my eyes.
I used live view so I could autofocus reliably. The camera was on a tripod. The first photo is just a small resize of the full scene. Focus point was on the Hellman's mayo.
The next two are 100% crops from the lower left area of the capture. I did not do any post processing, shot RAW, turned sharpening off completely in Lightroom and absolutely no noise reduction used. I am only posting the ISO2500 because the ISO1600 didn't show any real difference either.
DA 35mm/2.4 ISO 2500 F2.5 1/100
DA L 18-55 at 35mm ISO 2500 F4.5 1/25
The other two lenses (A 50/1.7 and DA L 55-300 at 50mm and 55mm respectively) appeared to show a bit more noise. However, I attributed this to the longer focal length. Maybe that's incorrect? If anyone wants to see those crops I can post them, but I didn't know if they were relevant to this discussion.
Anyway, if there are any differences in noise level in these photos, they appear very minor to me. Not sure what to make of this. Perhaps it was some placebo effect or the thrill of getting a new lens? Or maybe certain other conditions bring out what I was seeing more than this particular one.
But it was an interesting discussion and if you have any more thoughts, I'd love to read them! Thanks all!