[nis]

Hm, as I said IQ-wise you will gain hardly anything apart from less vignetting  with a 24-70 over your 18-105 which is quite a sharp lens. The Sigma will probably even be worse than your 18-105 while the Tamron will be more or less be the same (may be a tad sharper, but hardly anything you will notice in practice). You dont sound like you need the f/2.8 aperture either, so I would recommend you stick with your current setup. It is very good.

Yes.. I'm pretty satisfy with my kit lens, sharp enough for me.. but sometime their autofocus just miss to nail critical moment... actually you're right, I don't use wide aperture so often... most of the time I use some where between 5.6 to 8... I only use 3.5 (maximum aperture on kit lens at 18mm) when things getting to dark, and pumping ISO reaching the barrier of 3200