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.
The manufactures state a general working aperture for their TCs which is usually slower than what they actually will work with, depending on the lens in question. They are forced to in order to avoid numerous false advertising claims. I'm not saying that it will work with your lens, but neither am I saying that it won't. You really have to try it out, and if it won't AF in good light, then you either have to accept it as a MF converter for that particular lens, keep it for future use with a more suitable lens, or return it for a refund, accepting whatever charges might be appropriate.
A good example is the P F 1.7x AFA. Pentax states that it will only work with lenses with widest apertures of f2.8 or faster, yet I find it works well with the FA*300 f4.5, A 70-210 f4, and a number of other lenses slower than the what the specs allow.
Something that you might consider is a used Kenko 1.5x -- KEH as of this writing has a number of them in stock for good prices. They are a very reputable dealer. It might be worth a shot. . .