I have the original Tamron 28-200/3.5-5.6 autofocus which I
travelled the world with (30 countries and counting!) in the late
90's and it did a fab job with slide but I would say not all shots
were as sharp as they could be. To keep travel lite & handy it was
the best thing going. The newer models are better and focus closer.
I since bought the Tamron 28-300 (previous model) and it is a much
better lens (although to be fair the 28-200 has been updated a
couple of times since). As I have mentioned previously, if used
carefully (steady hand or tripod etc.) it will give excellent
result especially up to about 180mm -- at 300mm it is not as sharp
but first half of the zoom range is as sharp as pretty much
anything provided you stop down at least a stop.
Recently I took a family portrait at 100mm (x1.5) on a tripod at f8
and printed at 15"x11" that totally surprised me with sharpness.
Every blade of grass and every wrinkle

which was incredible
given there were 10+ people in the group.
Generally the two lenses I carry are the DA16-45 and the Tamron
28-300 (+sometimes the 50/1.4) --- any more and it becomes a hugh
production!
Have you looked at this site for lens MTF data (they stopped
testing in 2001 I think so no recent lenses on there but consistent
as they used the same equipment for all lenses)?
http://www.photodo.com/nav/prodindex.html
Also,
What about the Sigma 70-300 APO II ? AU$462 at centre.net.au
http://shop.centre.net.au/index.html?cat=&it=product&prid=001345