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

I bought the Tamron 18-270 (old version) 2 years ago together with my 60D and I have used it as my walkaround-lens every since. It has generally served me well, in 45 degrees C plus as well as -30 degrees minus, in deserts and in rain forests. I also believe for my kind of candid photography, this sort of all-range-lens will always have a place for all the times I would be too slow to change the lenses.

However, I have gradually become more displeased with the image quality, especially after I found out that my Powershot SX40, which i generally don't like image-quality-wise, produces far sharper images at the tele-end even when reducing the resolution from 18 MP to 12MP. Another problem is that while the tele-end of the Tamron is quite "blurry" even at smaller apertures, the other ranges are also only usable from about F/8, i.e. this lens is far too slow for many low light uses (and the IS not really good enough to counteract in all cases). You don't really want your DSLR being beaten by your backup P&S image-quality-wise!

Last summer I got the new version of the Tamron 70-300 IS and I have to say this lens blow the 18-270 out of the water and totally satisfies my requirements regarding sharpness, in the 70-300 range. It showed me for the first time what is really possible with an 18 MP DSLR image-quality-wise, compared to P&S.

All I need now is a lens covering the range from normal wide-angle (about 15-24; I also have the Tamron 10-24 which covers the extreme wide angle for me) to mid-range (close to or higher than 70). A lens that provides a similiar "sharpness boost" as going from the Tamron 18-270 to the Tamron 70-300 for the tele range AND allows me to use the lens in low light situations better (with bigger apertures and/or better IS).

I see there is a large variety of options, which I hope you can help me with assessing regarding my intended improvements:

- Tamron 24-70 F/2.8 IS: sounds like a big improvement, but a tad expensive for my taste (I'd prefer the $500 or lower range) and I've read the sample quality is inconsistent. Also: too little wide-angle on my crop camera

- Canon EF 24-105 1:4 L IS USM: only costs a little more than the Tamron and more useful tele range, but i have no idea how it fares quality-wise. Still generally too expensive and too little wide-angle.

- Canon EF-S 18-135 3.5-5.6 IS: Range sounds good, but I understand the image quality of this kit lens would not be an improvement really

- Canon EF-S 15-85mm 3.5-5.6 IS USM: covers from wide-angle to past 70mm, but quite expensive given the maximum apertures are not really an improvement.
- Canon EF-S 17-85 4.0-5.6 IS USM - Much cheaper than the above and also covers the complete range, but the maximum apertures are even worse. No idea about image quality.
- Tamron SP AF 17-50mm 2.8 Di II VC - This is the one I currently find most attractive given the price , the maximum apertures and the wide angle (keeping it all Tamrons would be a plus). Possible problem: I have 50-70mm uncovered without resorting to the Tamron 18-270 again.
So I'm interested to hear you opinion about these and other choices and how these would fare compared to my old Tamron 18-270, which provide the most improvement for the expense.