Found out throughout Camera Canada that the 1.4 tele made by Tamron only works with lens less than aperature 4.0. No other lens cuts it either. So far it does not appear that enhancment of the 18-250 is possible. Too bad I did not rechearch further before posting. I was looking for the magic answer.
that spec probably only means that it will only work in AF with a lens faster than f4. it may have nothing to do with the lens if you use it with a slower than f4 lens. if you find out differently, please post the results. i'm not quite sure. i've got an AF tamron 2x and it works with any lens i have.. however it does not AF with the kit lens which is the only AF i have.