I've shot 1/15 sec fairly often with decent results
there are a lot of factors that determine what i can shoot at though (lens size is a big one - no different then pre SR cameras this way)
on wider lenses i have no issue with the slower speeds (my DA 14 i could probably get even lower than this), it's influenced a lot by shooting technique as well
I have a habit at low speeds of forming a tripod with my arms and body and holding my breath during shutter releases developed from when i shot film. I also find it easier to hold my k7 with grip steadier than without. the extra weight helps. I have the opposite issue if the lens is heavy and it becomes front heavy. I've shot the canon OS lenses ages back and found them pretty effective as well.( aside from having sold them a friend had a couple with his A2E in the late 90's that i tried- almost got me to switch systems after trying them - price stopped me
)
I'm not sure the optical will give you 2 stops more than Sr, but it will on longer lenses make the viewfinder image sharper. Sr continually improves as does OS (sigma backwards engineered the Nikon system I think judging by the ongoing lawsuit between Nikon and them)