Tele-converters in general, including Tamron brand converters, are mainly intended for use with fixed focal length lenses and lenses that have a fast f value, such as our AF 90 mm f/2.8 (model 272E) and AF 180 mm f/3.5 (model B01).
If additional lens elements are introduced into an optical system, as done with the use of a tele-converter, the image quality of the whole system is negatively affected. As the image quality of a megazoom lens is already less then that of a professional and fast Tamron SP (zoom)lens, the final sharpness with the converter attached may be a lot less then without a teleconverter.
Less light enters the lens when using a tele-converter.
In the case of a 1,4x converter you lose one stop and with a 2x converter two stops. To assure correct functioning of the camera, lenses with an f value of minimum 4.0 are necessary. As most camera's can only have proper autofocus with an aperture up till F/5.6 or some models F/8.0.
The combined aperture of a lens + teleconverter must not be slower then this. So a 18-400mm with max F/6.3 at 400mm will get an effective F value of F/9.0 and even F/13. The camera can then only be used in manual focus mode!
ps
With some camera brands and models using the "Live View" mode will give some gain, as this way of focusing needs less light.