I've quite often used my 100-200 with the teleconverter, in good light, with very acceptable results.

I'm not sure why the lens geometry in the 250mm should make it unsuitable but it may be like the Zeiss Mutars for the Contax 35mm SLR lenses. They offer 2 different 2x converters, depending on the lenses being used though with some crossover of lens models, and a 1.4x which only suits a very limited selection of the glassware.