Frankly I'm skeptical anyone can get pictures that consistently show no movement at 1/15 of a second. Are any of these on the web where you can provide a link? Also, what size do you enlarge too? Movement gets worse the larger you blow up so what you think is OK on a light box quickly shows movement as you enlarge.
A I said I can shoot 1/30 no problems. 1/15 too if I brace myself a bit and breath easy with the WLF, resting the camera against my body. Obviously if the subject is moving (ie people) then that is no good but for static shots it is ok. Low shutter speeds are subjective, I have seen 1/8 shots from a friend's RZ67 which he handheld and they were sharp but I can't do it reliably. Other people need minimum 1/focal length or more as we all have different body types, technique and caffeine intake.
I get my film scanned by a lab and get 7000px on the wide end. I view files on a 27'' iMac and I printed a few to A4 size. I don't see camera shake at these conditions but then again I don't judge photos on absolute sharpness with a microscope. So it depends on your definition of usable/acceptable. I'll accept that if I blow it up to 40x30 it probably won't be critically sharp but I don't intend to. I don't post full resolution images online and have no intention to so I'm afraid web sized stuff won't tell you what you are looking for. Feel free to browse my site, panosvoudouris.com, the City shots are panf and acros at 1/15-1/60, last shot (arcade) was I think 1/4 (and it shows).