Sharper replacement for Tamron 18-270 (wide to mid range)?

As I mentioned, I'm leaning towards 17-50 due to its sharpness and attractive price, although after learning there is a VC and non-VC version and a lot of controversy about them, I'm in doubt again

My problem is this: one of the reasons I realized I need a replacement for the 18-270 in the midrange is my wish to shoot handheld in low-light - the last I tried this did not "exactly" work out as planned (and quite predictably, you may say). Thus I'm not so sure F/2.8 alone is helping me (in many occasions the depth of field is simply too small, i.e., when shooting indoors) Even though many people opine that IS is not really necessary for the range below 50mm, I can imagine wanting to shoot with shutter speeds below 1/40... I find I often end up there and below (although I don't know how many cases are due to the 18-270 forcing me to use F/8 - as I said - In some situations wider open may work if the lens is shaprer, in others not)

Is there any insight on recent improvements on the 17-50 VC, especially when used together with the 60D? (I read something about AF problems early on) Would this lens be as much an improvement as the mentioned 15-85 (which costs more than twice as much and thus is too expensive for me)?

As for the restricted upper end of the 17-50 (the gap between 50 and 70mm), this would be a less a problem for me - I find I use the range 18-50 on my 18-270 much more than any other range for all the cases I can't change to my 70-300. So it seems a better option for me to start "wide" - otherwise I always would have to carry my ultra-wide lens too (I can imagine in most cases it would be EITHER packing the ultra-wide OR the 17-50 -- I'm already carrying the 70-300 and 18-270 routinely now!)