welcome to the forum.
i'll agree with everything said here. i don't think i've ever been happy with a 2x + zoom lens.. a good one will cost a little less than a new lens.. i've got a tamron 2x mc4 and i'm happy happy with it on some lenses but never on a zoom. zooms usually are mad of of 14-18 pieces of glass.. you'll be adding 4-7 more pieces when using a TC. .. the loss of aperture is enough alone. ck what you'll see by using the stop down feature of the camera. put the lens on 250mm and set the aperture to f16, hit the stop down button and you'll see the brightest you'll ever get with that lens.
just my 2¢,