focus speed tamron 70-200 f2.8
I used that non-VC Tamron a lot while working for a media house and I was not exactly impressed - either by image quality or focusing speed.
This is of course a very subjective point - your standards of speed and quality might be different than mine.
Btw, I've written a short review about that lens:
EDIT: Sorry, forgot to say that focus speed compared to the 18-105 is about the same probably (or even a hint slower), although in lower light the faster lens will give you less hunting, obviously
|IMG_8168ABCD by citori525|
|McKinley meadow by TimR32225|
from Natural meadows
|_DSC2146 by jerste|
from Helios-44 II
|Leopoldsteinersee by RaCor|
from Landscape - Colour #3