I personally don't think so, as for the purpose of getting this next purchase (Tamon Non VC/Sigma 17-50) is for low light shooting... *I think I need some help/advice here*
Personally, I'm not very particular about IQ (e.g. Chromatic distortion, Barrel Distortion etc) as I don't pixel peep that much (I only "peep" if I crop too much). Well, the few times I complaint about the IQ of photos is 
- when using my tele zoom (55-250) on the far end (250mm) in the Singapore Zoo, where it gets really soft and the flaws of the lens starts to show (esp in lesser light. overcasts and FURRY CREATURES!). *Heck, I didn't even notice the CA on my 18-55..if there's any, probably due to the blessing that I've yet to seen a L in action*
 
- Too high ISO (e.g. 1600) used and the photo turned out noisy but aperture already at wide open on that lens (e.g. 18-55 @ 55mm f/5.6).
 
Stuffs that I've noticed for quite sometime already is, for my kit 18-55 f3.5-5.6, most/many of my shots are done at the both ends (18mm and 55mm), aperture wide open at high ISO of 800-1600. Stuffs that I shoot under these conditions are like objects/food photos in a restaurant like environment. 
Many times my shots (candid shots) turn out underexposed (even @ ISO1600), this is leading me to believe that I need a lens with wider aperture as I believe that using flash will "kill" off the ambiance, still I will use my ext.flash if ambiance light is really not enough). At the same time, I find prime lens abit restrictive for my liking (that said, my only prime lens experience is with my 50 f1.8, which wasn't exactly what I would call pleasant).
Below are some examples of what I considered to be low light...
*Indoor Candle Light*
*Indoor Hotel wedding dinner table shot*
*Outdoor Night lantern*
*Outdoor Lightings*
*btw, low light landscape will not be taken into consideration for this lens, cos I will be on tripod and "any" lens that have the required focal length will do. Focusing wise, at the most MF lor...not very hard on tripod, but 
I find it a challenge when handholding it and MFing at the same time*
I guess the burning question now is
- do I need the f/2.8 or it's just my camera's ISO limit. My heart was pretty much going in for the Sigma 17-50 cos of the OS and f2.8. But yet at the same time confused which is the problem.
- Will the AF of Sigma "hunt" in this kind of lighting? The last thing I want is the AF keep hunting when I want to do a group photo in lighting like in shot #2
Any advices?